QtUpdateCompilerOptions massively bloats compile times by multiple minutes, for projects with a very large number of files

[DavidJCobb]

Describe the bug
The QtUpdateCompilerOptions build target is adding anywhere from two to four entire minutes to every build.

Using vcperf produces an ETL file which shows MSBuild open, using only a moderate amount of CPU, for the entire duration of the lag spike, after which actual compiling begins and completes near-instantly. Vcperf is incapable of displaying the cause of the lag; I was lucky to find the appropriate logging option in Visual Studio (after enabling basically every logging-related environment variable and UI option that Microsoft ever added to VS over the last ten years).

I believe the VS logging option that allowed me to narrow this down to Qt VS Tools was: Options -> Projects and Solutions -> Build and Run -> MSBuild project build output verbosity, set to value Diagnostic.

My current project/solution is not public. I would rather not have to provide the whole thing at this time, if that can be avoided.

Expected behavior
It shouldn't take four entire minutes just for Qt VS Tools to tamper with command line options before Visual Studio can even start compiling anything. What in the blazes are you doing?

Desktop (please complete the following information):

• Qt VS Tool Version 3.5.0
• Visual Studio version VS2026

Additional context
Excerpts from build output produced after making a single-line change to my code, with the aforementioned MSBuild diagnostic setting enabled:

8>01:21:04.105     1>Target "QtUpdateCompilerOptions: (TargetId:2471)" in file "C:\Users\Cobb\AppData\Local\QtMsBuild\qt_build.targets" from project "D:\UserContent\Cobb\Documents\Development\Modding\Skyrim\DovahKit\build\DovahKit\DovahKit\DovahKit.vcxproj" (target "FixupCLCompileOptions" depends on it):
8>01:24:36.583     1>Done building target "QtUpdateCompilerOptions" in project "DovahKit.vcxproj".: (TargetId:2471)

and in here, DovahKit.vcxproj is the affected project; two of its dependency projects are included for comparison's sake:

D:\UserContent\Cobb\Documents\Development\Modding\Skyrim\DovahKit\build\DovahKit\Lua\Lua.vcxproj   4 calls
8>                  4 ms  GetTargetPath                              1 calls
8>                  1 ms  GetNativeManifest                          1 calls
8>                 48 ms  GetProjectInfoForReference                 1 calls
8>                  5 ms  GetCopyToOutputDirectoryItems              1 calls
8>       85 ms  D:\UserContent\Cobb\Documents\Development\Modding\Skyrim\DovahKit\build\DovahKit\VulkanMemoryAllocator\VulkanMemoryAllocator.vcxproj   4 calls
8>                  5 ms  GetTargetPath                              1 calls
8>                  0 ms  GetNativeManifest                          1 calls
8>                 75 ms  GetProjectInfoForReference                 1 calls
8>                  5 ms  GetCopyToOutputDirectoryItems              1 calls
8>    287728 ms  D:\UserContent\Cobb\Documents\Development\Modding\Skyrim\DovahKit\build\DovahKit\DovahKit\DovahKit.vcxproj   3 calls
8>              287725 ms  Build                                      1 calls
8>                  3 ms  QtVarsRead                                 2 calls
8>
8>Target Performance Summary:
8>        0 ms  QtQmlSetModified                           1 calls
8>        0 ms  ComputeManifestGeneratedLinkerInputs       1 calls
8>        0 ms  ComputeRCGeneratedLinkInputs               1 calls
8>        0 ms  ComputeRCOutputs                           1 calls
8>        0 ms  BeforeLink                                 1 calls
8>        0 ms  BuildLinkTraverse                          1 calls
8>        0 ms  BuildCompile                               1 calls
8>        0 ms  _BuildCompileAction                        1 calls
8>        0 ms  AfterBuildCompileEvent                     1 calls
8>        0 ms  _ResourceCompile                           1 calls
8>        0 ms  AfterResourceCompile                       1 calls
8>        0 ms  SelectResourceCompile                      1 calls
8>        0 ms  MakeDirsForResourceCompile                 1 calls
8>        0 ms  BeforeResourceCompile                      1 calls
8>        0 ms  ComputeLinkInputsFromProject               1 calls
8>        0 ms  _ClCompile                                 1 calls
8>        0 ms  ForciblyDisableModules                     1 calls
8>        0 ms  QtQml                                      1 calls
8>        0 ms  QtQmlCreateItems                           1 calls
8>        0 ms  QtQmlCache                                 1 calls
8>        0 ms  QtQmlCacheItem                             1 calls
8>        0 ms  QtQmlCacheLoader                           1 calls
8>        0 ms  QtRccFilter                                1 calls
8>        0 ms  QtRccFilterSetModified                     1 calls
8>        0 ms  QtRccPrepare                               3 calls
8>        0 ms  QtRccInit                                  1 calls
8>        0 ms  QtTranslation                              1 calls
8>        0 ms  QtGetDefaultClCompile                      1 calls
8>        0 ms  AddLibrariesToProjectsInfoForReference     7 calls
8>        0 ms  AfterClCompile                             1 calls
8>        0 ms  ResolvedLinkLib                            8 calls
8>        0 ms  ComputeResolveLinkObj                      1 calls
8>        0 ms  QtRccPass2                                 1 calls
8>        0 ms  IncrementalClean                           1 calls
8>        0 ms  AfterBuild                                 1 calls
8>        0 ms  BuildLink                                  1 calls
8>        0 ms  _BuildLinkAction                           1 calls
8>        0 ms  PostBuildEvent                             1 calls
8>        0 ms  PrepareForRun                              1 calls
8>        0 ms  CopyFilesToOutputDirectory                 1 calls
8>        0 ms  _CheckForCompileOutputs                    1 calls
8>        0 ms  _CopySourceItemsToOutputDirectory          1 calls
8>        0 ms  _Deploy                                    1 calls
8>        0 ms  _Appverifier                               1 calls
8>        0 ms  CreateSatelliteAssemblies                  1 calls
8>        0 ms  _GenerateSatelliteAssemblyInputs           1 calls
8>        0 ms  QtRccPass2_GetObjs                         1 calls
8>        0 ms  MakeDirsForFxc                             1 calls
8>        0 ms  _BscMake                                   1 calls
8>        0 ms  MakeDirsForBscMake                         1 calls
8>        0 ms  ComputeCLCompileGeneratedSbrFiles          1 calls
8>        0 ms  _XdcMake                                   1 calls
8>        0 ms  ResolvedXDCMake                            1 calls
8>        0 ms  _Manifest                                  1 calls
8>        0 ms  _ALink                                     1 calls
8>        0 ms  _Link                                      1 calls
8>        0 ms  QtNatvis                                   1 calls
8>        0 ms  PreLinkEvent                               1 calls
8>        0 ms  PrepareResourceNames                       1 calls
8>        0 ms  CreateCustomManifestResourceNames          1 calls
8>        0 ms  QtRccPass2_Link                            1 calls
8>        0 ms  SatelliteDllsProjectOutputGroup            1 calls
8>        0 ms  AddDeployRecipeFileToProjectsInfoForReference   7 calls
8>        0 ms  Build                                      1 calls
8>        0 ms  SelectCustomBuild                          1 calls
8>        0 ms  PrepareProjectReferences                   8 calls
8>        0 ms  _RemoveNameMetadataFromProjectReferenceItems   1 calls
8>        0 ms  BeforeClCompile                            1 calls
8>        0 ms  AddExternalIncludDirectoriesToPaths        8 calls
8>        0 ms  GetTargetPath                              7 calls
8>        0 ms  BuildGenerateSources                       1 calls
8>        0 ms  _BuildGenerateSourcesAction                1 calls
8>        0 ms  AfterBuildGenerateSourcesEvent             1 calls
8>        0 ms  AfterBuildGenerateSources                  1 calls
8>        0 ms  _Midl                                      1 calls
8>        0 ms  AfterMidl                                  1 calls
8>        0 ms  MakeDirsForMidl                            1 calls
8>        0 ms  _Xsd                                       1 calls
8>        0 ms  CopyFileToFolders                          1 calls
8>        0 ms  ComputeCustomBuildOutput                   1 calls
8>        0 ms  AddPublicBMIsToProjectsInfoForReference    7 calls
8>        0 ms  _SelectedFiles                             1 calls
8>        0 ms  PreBuildEvent                              1 calls
8>        0 ms  BeforeBuildGenerateSources                 1 calls
8>        0 ms  BuildGenerateSourcesTraverse               1 calls
8>        0 ms  ResolveReferences                          8 calls
8>        0 ms  AfterResolveReferences                     8 calls
8>        0 ms  ExpandSDKReferences                        8 calls
8>        0 ms  ResolveSDKReferences                       8 calls
8>        0 ms  GetFrameworkPaths                          8 calls
8>        0 ms  FindReferenceAssembliesForReferences       1 calls
8>        0 ms  DisableProjectReferenceRecursionForClInputs   7 calls
8>        0 ms  BuildCompileTraverse                       1 calls
8>        0 ms  GetPublicIncludeDirectories                7 calls
8>        0 ms  _AddOutputPathToGlobalPropertiesToRemove   8 calls
8>        0 ms  BeforeResolveReferences                    8 calls
8>        0 ms  ATGEnsureShaders                           1 calls
8>        0 ms  AddPublicIncludeDirectoriesToProjectsInfoForReference   7 calls
8>        0 ms  IgnoreJavaScriptOutputAssembly             8 calls
8>        1 ms  _PrepareForBuild                           1 calls
8>        1 ms  SplitResourcesByCulture                    1 calls
8>        1 ms  ComputeManifestInputsTargets               1 calls
8>        1 ms  QtRccOverrideCpp                           3 calls
8>        1 ms  ResolveAssemblyReferences                  8 calls
8>        1 ms  ComputeMIDLGeneratedCompileInputs          8 calls
8>        1 ms  QtMocUpdate                                1 calls
8>        1 ms  AfterLink                                  1 calls
8>        1 ms  GetProjectInfoForReference                 7 calls
8>        1 ms  QtQmlStaticGenerateImportFile              1 calls
8>        1 ms  ComputeReferenceLinkInputs                 1 calls
8>        1 ms  QtQmlCachePrepare                          3 calls
8>        1 ms  QtVars                                     1 calls
8>        1 ms  _PopulateCommonStateForGetCopyToOutputDirectoryItems   8 calls
8>        1 ms  ComputeCLGeneratedLinkInputs               1 calls
8>        1 ms  FinalizeBuildStatus                        1 calls
8>        1 ms  MakeDirsForXdcMake                         1 calls
8>        1 ms  ComputeLegacyManifestEmbedding             1 calls
8>        2 ms  SetCABuildNativeEnvironmentVariables       8 calls
8>        2 ms  _CleanGetCurrentAndPriorFileWrites         1 calls
8>        2 ms  GetResolvedLinkLibs                        7 calls
8>        2 ms  ComputeReferenceCLInput                    1 calls
8>        2 ms  QtRccSetDependencies                       3 calls
8>        2 ms  GetResolvedWinMD                           8 calls
8>        3 ms  GetCopyToOutputDirectoryXamlAppDefs        8 calls
8>        3 ms  _SplitProjectReferencesByFileExistence     8 calls
8>        3 ms  GetReferenceAssemblyPaths                  8 calls
8>        3 ms  DoLinkOutputFilesMatch                     1 calls
8>        3 ms  _PrepareForReferenceResolution             8 calls
8>        3 ms  QtUpdateLinkOptions                        1 calls
8>        3 ms  ComputeLinkSwitches                        1 calls
8>        3 ms  QtVarsRead                                 1 calls
8>        3 ms  BuiltProjectOutputGroup                    1 calls
8>        3 ms  QtVarsLoad                                 1 calls
8>        3 ms  GetTargetPathWithTargetPlatformMoniker     7 calls
8>        3 ms  GetNativeManifest                          7 calls
8>        4 ms  _CheckWindowsSDKInstalled                  8 calls
8>        4 ms  QtRccSetModified                           3 calls
8>        4 ms  ComputeCLCompileGeneratedXDCFiles          1 calls
8>        4 ms  InitializeBuildStatus                      1 calls
8>        4 ms  _GetCopyToOutputDirectoryItemsFromThisProject   8 calls
8>        5 ms  QtUicInit                                  1 calls
8>        5 ms  MakeDirsForLink                            1 calls
8>        5 ms  SetTelemetryEnvironmentVariables           1 calls
8>        6 ms  SelectClCompile                            1 calls
8>        6 ms  CustomBuild                                1 calls
8>        7 ms  ResourceCompile                            1 calls
8>        7 ms  RegisterOutput                             1 calls
8>        8 ms  LogAllBuildInfo                            1 calls
8>       11 ms  _GetProjectReferenceTargetFrameworkProperties   8 calls
8>       11 ms  AssignProjectConfiguration                 8 calls
8>       13 ms  QtMocInit                                  1 calls
8>       13 ms  AssignTargetPaths                          8 calls
8>       14 ms  MakeDirsForCl                              3 calls
8>       14 ms  ComputeCLInputPDBName                      8 calls
8>       14 ms  CreateRecipeFile                           1 calls
8>       19 ms  QtRcc                                      3 calls
8>       20 ms  _CheckForInvalidConfigurationAndPlatform   8 calls
8>       21 ms  SetModuleDependencies                      3 calls
8>       21 ms  SetCppLanguageStandardCapabilities         8 calls
8>       22 ms  ContentFilesProjectOutputGroup             1 calls
8>       23 ms  QtUicPrepare                             167 calls
8>       28 ms  QtUicSetModified                         167 calls
8>       28 ms  QtVarsPrepare                              1 calls
8>       32 ms  ComputeStdModulesCompileInputs             8 calls
8>       39 ms  ComputeCLOutputs                           1 calls
8>       43 ms  SetBuildDefaultEnvironmentVariables        8 calls
8>       46 ms  WarnCompileDuplicatedFilename              8 calls
8>       48 ms  QtPrepare                                  1 calls
8>       49 ms  QtMocPrepare                             332 calls
8>       60 ms  QtMocSetModified                         332 calls
8>       70 ms  PrepareForBuild                            8 calls
8>       82 ms  GetReferencedVCProjectsInfo                8 calls
8>       86 ms  TlogCleanup                                1 calls
8>      122 ms  QtQmlStaticGatherQmlPaths                  1 calls
8>      144 ms  QtHash                                     1 calls
8>      227 ms  QtWork                                     1 calls
8>      230 ms  QtAddCompilerSources                       1 calls
8>      280 ms  QtWorkPrepare                            502 calls
8>      356 ms  ResolveProjectReferences                   8 calls
8>      387 ms  QtUic                                    167 calls
8>      447 ms  QtMocOverrideCpp                         332 calls
8>      617 ms  FixupCLCompileOptions                      8 calls
8>      686 ms  QtDeploy                                   1 calls
8>     2655 ms  QtMoc                                    332 calls
8>     7678 ms  Link                                       1 calls
8>     9218 ms  _GetCopyToOutputDirectoryItemsFromTransitiveProjectReferences   8 calls
8>     9234 ms  GetCopyToOutputDirectoryItems              8 calls
8>    51548 ms  ClCompile                                  1 calls
8>    212478 ms  QtUpdateCompilerOptions                    1 calls
8>
8>Build succeeded.

The source code for the QtUpdateCompilerOptions build target is here. It's configured to produce log output if QtOptionsBuildLog (apparently corresponding to the "Show in Build Log" option in Qt settings) is turned on. The resulting log output, from a later build, was:

8>11:09:24.311     1>Target "QtUpdateCompilerOptions: (TargetId:2471)" in file "C:\Users\Cobb\AppData\Local\QtMsBuild\qt_build.targets" from project "D:\UserContent\Cobb\Documents\Development\Modding\Skyrim\DovahKit\build\DovahKit\DovahKit\DovahKit.vcxproj" (target "FixupCLCompileOptions" depends on it):
8>                      Qt - Additional Compiler Options (TaskId:3146)
8>                         [dovahscript\wrappers\form\dialogue_branch.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:3147)
8>                         [dovahscript\wrappers\form\navmesh.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:3148)
8>                         [dovahscript\wrappers\form\navmesh\collection_base_forms.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:3149)
8>                         [dovahscript\wrappers\form\navmesh\collection_door_links.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:3150)
8>                         [dovahscript\wrappers\form\navmesh\collection_triangles.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:3151)

...Similar lines are produced for every single cpp file in the project, including files produced by Qt codegen, with the results literally exceeding this issue form's character limit...

8>                         [xaudio2\simple_fuz_sound.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4503)
8>                         [xaudio2\xwma_file_info.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4504)
8>                         [DefaultClCompile]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4505)
8>                         [x64\Debug\qt\rcc\qrc_all_compiled.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4506)
8>                         [x64\Debug\qt\rcc\qrc_resources.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4507)
8>                         [x64\Debug\qt\rcc\qrc_widget-assets.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4508)
8>                         [x64\Debug\qt\moc\moc_DKAudioWidgetSimple.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4509)
8>                         [x64\Debug\qt\moc\moc_simple_fuz_sound.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4510)
8>                         [x64\Debug\qt\moc\moc_DKTopicOrSubtypePickerModel.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4511)
8>                         [x64\Debug\qt\moc\moc_DKMagicEffectListModel.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4512)
8>                         [x64\Debug\qt\moc\moc_QuestTabScenes.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4513)

...snip: more MOC...

8>                         [x64\Debug\qt\moc\moc_shout_word.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4835)
8>                         [x64\Debug\qt\moc\moc_form_table.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4836)
8>                         [x64\Debug\qt\moc\moc_QLinedTreeView.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4837)
8>                         [x64\Debug\qt\moc\moc_object_window.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4838)
8>                         [x64\Debug\qt\moc\moc_core.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4839)
8>                         [x64\Debug\qt\moc\moc_main_window.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:4840)
8>11:12:49.829     1>Done building target "QtUpdateCompilerOptions" in project "DovahKit.vcxproj".: (TargetId:2471)

The total number of CPP files that this build target is processing is 1694.

[DavidJCobb]

Possible similar problem reports:

• Feb 12 2026, redirected to Qt's issue tracker
• Mar 26 2026

[DavidJCobb]

I managed to find the installed props/targets file at C:\Users\<me>\AppData\Local\QtMsBuild\qt_build.props. I tested out some edits to that file, though Qt VS Tools re-downloads the file on startup so I had to wait until after restarting VS.

MSBuild doesn't appear to have any built-in tasks that can be used to print the current time. However, the PowerShell Get-Date command can be used to get, format, and print the current timestamp, and can be invoked using an <Exec /> task. I decided to benchmark each individual step of Qt VS Tools's faulty target, while compiling just one .cpp file (by right-clicking it in the Solution Explorer). First, I prepended the following code to the target:

<PropertyGroup>
<PowerShellExe Condition=" '$(PowerShellExe)'=='' "> "%WINDIR%\System32\WindowsPowerShell\v1.0\powershell.exe"</PowerShellExe>
<PowerShellPrintTime Condition=" '$(PowerShellPrintTime)'=='' ">$(PowerShellExe) -NonInteractive -executionpolicy Unrestricted -command "Get-Date -Format 'HH:mm:ss.ffffff'"</PowerShellPrintTime>
</PropertyGroup>

Then, I added code like the following between each step of the target:

<Exec EchoOff="true" Command="$(PowerShellPrintTime)" />
<Message Text="QtUpdateCompilerOptions bench 01..." />

The resulting output was:

1>                     23:00:41.729524 (TaskId:92)
1>                     QtUpdateCompilerOptions bench 01... (TaskId:93)
1>                     23:00:41.909421 (TaskId:94)
1>                     QtUpdateCompilerOptions bench 02... (TaskId:95)
1>                     23:00:42.097467 (TaskId:96)
1>                     QtUpdateCompilerOptions bench 03... (TaskId:97)
1>                     23:00:42.277395 (TaskId:98)
1>                     QtUpdateCompilerOptions bench 04... (TaskId:99)
1>                     23:00:43.078280 (TaskId:101)
1>                     QtUpdateCompilerOptions bench 05... (TaskId:102)
1>                     23:00:43.270261 (TaskId:103)
1>                     QtUpdateCompilerOptions bench 06... (TaskId:104)
1>                     23:00:43.477165 (TaskId:105)
1>                     QtUpdateCompilerOptions bench 07... (TaskId:106)
1>                     23:00:43.656857 (TaskId:107)
1>                     QtUpdateCompilerOptions bench 08... (TaskId:108)
1>                     23:00:44.366323 (TaskId:110)
1>                     QtUpdateCompilerOptions bench 09... (TaskId:111)
1>                     23:00:44.649953 (TaskId:112)
1>                     QtUpdateCompilerOptions bench 10... (TaskId:113)
1>                     23:01:10.051660 (TaskId:114)
1>                     QtUpdateCompilerOptions bench 11... (TaskId:115)
1>                     23:02:00.633809 (TaskId:116)
1>                     QtUpdateCompilerOptions bench 12... (TaskId:117)
1>                     23:02:01.594723 (TaskId:118)
1>                     QtUpdateCompilerOptions bench 13... (TaskId:119)
1>                     23:02:01.833133 (TaskId:120)
1>                     QtUpdateCompilerOptions bench 14... (TaskId:121)
1>                      Qt - Additional Compiler Options (TaskId:122)
1>                         [dovahscript\wrappers\form\dialogue_branch.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:123)

-snip-

1>                         [xaudio2\xwma_file_info.cpp]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:1482)
1>                         [DefaultClCompile]: -Zc:rvalueCast -Zc:strictStrings -Zc:throwingNew -Zc:referenceBinding -Zc:__cplusplus (TaskId:1483)
1>                     23:02:02.132467 (TaskId:1484)
1>                     QtUpdateCompilerOptions bench 15... (TaskId:1485)
1>                     23:02:52.780680 (TaskId:1486)
1>                     QtUpdateCompilerOptions bench 16... (TaskId:1487)

So it's roughly 25 seconds for the following instruction ("bench 11", beginning at 23:00:44 and completing at 23:01:10):

    <!-- Calculate option id: token without leading '/' or '-', and without trailing '-' -->
    <ItemGroup>
      <ClOptions>
        <OptionId>$([System.String]::Copy('%(Value)').Substring(1).TrimEnd('-'))</OptionId>
      </ClOptions>
    </ItemGroup>

50 seconds for the following instruction ("bench 12", beginning at 23:01:10 and completing at 23:02:00):

    <!-- Split into list of Qt options and list of project options -->
    <ItemGroup>
      <QtOptions
        Include="@(ClOptions->'%(Item)')"
        Condition="'%(ClOptions.Name)' == 'QtOptions'"/>
      <ProjectOptions
        Include="@(ClOptions->'%(Item)')"
        Condition="'%(ClOptions.Name)' == 'ProjectOptions'"/>
    </ItemGroup>

And 50 seconds for the following instruction ("bench 16", beginning at 23:02:02 and completing at 23:02:52):

    <!-- Clean-up -->
    <PropertyGroup>
      <regex/>
      <options/>
      <cl_cmd/>
    </PropertyGroup>
    <ItemGroup>
      <ClCompile_CmdLine Remove="@(ClCompile_CmdLine)"/>
      <ClOptions Remove="@(ClOptions)"/>
      <ProjectOptions Remove="@(ProjectOptions)"/>
      <QtOptions Remove="@(QtOptions)"/>
      <ClCompile Remove="DefaultClCompile"/>
    </ItemGroup>

I'm still looking into MSBuild syntax, so at this time, I don't have any insight to offer on how these three operations are doing what they're doing.

[DavidJCobb]

I've managed to resolve the lag by replacing the target with the following code. However, I have to do this every time I restart VS, since Qt VS Tools re-downloads the targets file on startup.

  <Target Name="QtUpdateCompilerOptions"
          DependsOnTargets="QtAddCompilerSources;QtPrepareResourceCompilerDefines"
          BeforeTargets="ClCompile;ResourceCompile">

    <ItemGroup>
      <QtIncludePath Include="$(Qt_INCLUDEPATH_);$(ProjectDir)" />
    </ItemGroup>
    <PropertyGroup>
      <QtIncludePath>@(QtIncludePath->Distinct())</QtIncludePath>
    </PropertyGroup>
    <ItemGroup>
      <ClCompile>
        <AdditionalIncludeDirectories Condition="'$(QtIncludePath)' != ''"
        >$(QtIncludePath);%(ClCompile.AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
        <PreprocessorDefinitions Condition="'$(Qt_DEFINES_)' != ''"
          >$(Qt_DEFINES_);%(ClCompile.PreprocessorDefinitions)</PreprocessorDefinitions>
        <PreprocessorDefinitions Condition="'$(QtQMLDebugEnable)' == 'true'"
          >QT_QML_DEBUG;%(ClCompile.PreprocessorDefinitions)</PreprocessorDefinitions>
        <LanguageStandard Condition="'$(Qt_STDCPP_)' != ''
                          AND '%(ClCompile.LanguageStandard)' == 'Default'"
          >$(Qt_STDCPP_)</LanguageStandard>
        <RuntimeLibrary Condition="'$(Qt_RUNTIME_)' != ''
                          AND '%(ClCompile.RuntimeLibrary)' == ''"
          >$(Qt_RUNTIME_)</RuntimeLibrary>
      </ClCompile>
      <ResourceCompile>
        <AdditionalIncludeDirectories Condition="'$(QtIncludePath)' != ''"
          >$(QtIncludePath);%(ResourceCompile.AdditionalIncludeDirectories)</AdditionalIncludeDirectories>
        <PreprocessorDefinitions Condition="'$(QtResourceCompilerDefines)' != ''"
          >$(QtResourceCompilerDefines);%(ResourceCompile.PreprocessorDefinitions)</PreprocessorDefinitions>
      </ResourceCompile>
    </ItemGroup>

    <!-- Command line parser regex -->
    <!-- Exclude non-switch tokens, i.e. tokens that do not start with '/' or '-' -->
    <!-- Exclude any trailing dashes as well -->
    <PropertyGroup>
      <regex>[-/]("[^"]*"|[^\s]+)-*</regex>
      <options>$(Qt_CL_OPTIONS_)</options>
    </PropertyGroup>

    <!-- Parse compiler options from Qt -->
    <PropertyGroup>
      <cl_cmd
>$([System.Text.RegularExpressions.Regex]::Matches('$(options)', '$(regex)'))</cl_cmd>
    </PropertyGroup>

    <!-- Calculate command line for each source file in the project -->
    <QtRunTask
      Items="@(ClCompile)"
      AssemblyPath="$(VCTargetsPath)\Microsoft.Build.CPPTasks.Common.dll"
      TaskName="Microsoft.Build.CPPTasks.CLCommandLine"
      TaskInput="Sources"
      TaskOutput="CommandLines"
      NewMetadata="ProjectOptions">
      <Output TaskParameter="Result" ItemName="ClCompile_CmdLine" />
    </QtRunTask>

    <!-- Append excluded Qt options to calculated command line -->
    <ItemGroup>
      <ClCompile_CmdLine Condition="'$(QtExcludedOptions)' != ''">
        <ProjectOptions>%(ProjectOptions) $(QtExcludedOptions)</ProjectOptions>
      </ClCompile_CmdLine>
    </ItemGroup>

    <!-- Parse compiler command line of each source file -->
    <ItemGroup>
      <ClCompile_CmdLine>
        <ProjectOptions
>$([System.Text.RegularExpressions.Regex]::Matches('%(ProjectOptions)','$(regex)'))</ProjectOptions>
      </ClCompile_CmdLine>
    </ItemGroup>

    <!-- Add (previously parsed) Qt options to each source file -->
    <ItemGroup>
      <ClCompile_CmdLine>
        <QtOptions>$(cl_cmd)</QtOptions>
      </ClCompile_CmdLine>
    </ItemGroup>

    <!-- Result of parsing command lines, per source file:
          * ClCompile_CmdLine ::= (Item x ProjectOptions x QtOptions), where:
             * Item ::= source file
             * ProjectOptions ::= list of tokens from the compiler command line for Item
             * QtOptions ::= list of tokens from the Qt additional options -->

    <!-- Flatten results into a list of tokens per source file:
          * ClOptions ::= (Item x Name x Value), where:
             * Item  ::= source file
             * Name  ::= token origin: from compiler command line or from Qt
             * Value ::= token value -->
    <Flatten Items="@(ClCompile_CmdLine)" Metadata="ProjectOptions;QtOptions">
      <Output TaskParameter="Result" ItemName="ClOptions" />
    </Flatten>

    <!-- Calculate option id: token without leading '/' or '-', and without trailing '-' -->
    <ItemGroup>
      <ClOptions>
        <OptionId>$([MSBuild]::ValueOrDefault('%(Value)',''))</OptionId>
      </ClOptions>
    </ItemGroup>

    <!-- Split into list of Qt options and list of project options -->
    <ItemGroup>
      <QtOptions
        Include="@(ClOptions->'%(Item)')"
        Condition="'%(ClOptions.Name)' == 'QtOptions'"/>
      <ProjectOptions
        Include="@(ClOptions->'%(Item)')"
        Condition="'%(ClOptions.Name)' == 'ProjectOptions'"/>
    </ItemGroup>

    <!-- Find conflicting options, i.e. defined both in Qt options and project options -->
    <ItemGroup>
      <QtOptions
        Condition="'@(QtOptions)' != ''
               AND '@(ProjectOptions)' != ''
               AND '%(Item)' != ''
               AND '%(OptionId)' != ''">
        <Conflict>true</Conflict>
      </QtOptions>
    </ItemGroup>

    <!-- Set additional compiler options for all source files -->
    <ItemGroup>
      <ClCompile Condition="'%(Identity)' != '' AND '%(QtOptions.Conflict)' != 'true'">
        <AdditionalOptions
          >@(QtOptions->'%(Value)', ' ') @(ClCompile->'%(AdditionalOptions)')</AdditionalOptions>
      </ClCompile>
      <ClCompile Condition="'%(AdditionalOptions)' != ''">
        <AdditionalOptions
          >$([System.String]::Copy('%(AdditionalOptions)').Trim())</AdditionalOptions>
      </ClCompile>
    </ItemGroup>

    <!-- Print result to build log, if requested -->
    <Message
      Condition="'$(QtOptionsBuildLog)' == 'true'"
      Importance="High"
      Text=" Qt - Additional Compiler Options"/>
    <Message
      Condition="'$(QtOptionsBuildLog)' == 'true'
             AND '%(Identity)' != '' AND '%(QtOptions.Conflict)' != 'true'"
      Importance="High"
      Text="    [%(Identity)]: @(QtOptions->'%(Value)', ' ')"/>

    <!-- Clean-up -->
    <PropertyGroup>
      <regex/>
      <options/>
      <cl_cmd/>
    </PropertyGroup>
    <ItemGroup>
      <ClCompile_CmdLine Remove="@(ClCompile_CmdLine)"/>
      <ClOptions Remove="@(ClOptions)"/>
      <ProjectOptions Remove="@(ProjectOptions)"/>
      <QtOptions Remove="@(QtOptions)"/>
      <ClCompile Remove="DefaultClCompile"/>
    </ItemGroup>
  </Target>

Instead of capturing every space-separated token in the command line by regex, and then trying to filter out anything that isn't a command line switch, we instead just do the filtering within that initial regex and only ever capture the command line switches in the first place. This reduces the total execution time for this task to about 7 seconds, which is a colossal improvement. I no longer dread running a build.

I can't say I know precisely why Qt's current approach can break MSBuild so badly. I suspect it scales with the number of files in a project, because MSBuild would be executing all of this string manipulation and so on for every file in the build. My affected project is... goodness, this is the first time I've checked -- over 4500 files, without paying attention to specific file types/extensions.