Slow compilation time because of a lot of MSBuild GetReferencesVCProjectInfo

I'm experiencing really slow compilation time in my solution. More exactly for each and every compilations (whole solution, or single file) it takes several seconds (usually ~10 seconds) before it starts to compile a file).

I've used Cpp Build Insights (Introducing C++ Build Insights - C++ Team Blog (microsoft.com)) to track what's going on.

I've noticed that the before it starts to compile any file, it spends a lot of time in MSBuild tasks to gather information about referenced projects (GetReferencedVCProjectInfo). And it happens only with Qt projects.

Sample code

To go further, I've created 2 use cases solution very similar:

• A console application with a main.cpp
• A bunch of libraries projects containing "a lot" of dummy files (.h / .cpp)
• The console application references all libraries, library 1 references libraries from 2 to 7, library 2 references libraries from 3 to 7, etc... to create a dependency graph
• All projects include some property sheets (.props)

All projects are created with the default VS templates. The 2 use cases are:

• Regular VisualC++ projects without Qt
• Qt projects

You can find these solutions attached to this ticket (Perf_WithQt, PerfWithoutQt).

Measurements

The goal was to measure the compilation of the console application main.cpp file only (not the whole solution).

Of course, the content of the 2 main files are not the same, but the problem doesn't come from the content of these files, it comes from the build system.

Now I focus on the Qt solution only. I've tested step by step like if I was building the solution from scratch:

1. I create the QtConsoleApplication only, without any referenced project, without property sheet
2. I add the library projects, with the referenced projects, without property sheet, without source files (empty project)
3. I add the source files contained in the 'NewFolder1' sub folder for each libraries
4. I add the property sheet Project.props on the console application project only
5. I add the property sheet Project.props on all libraries projects

So basically, to compile a single file, it's 10 times slower in this example.

This issue seems to be once per project. So for example if I notice a 10 seconds overhead to compile a single file for a particular project, I notice the same 10 seconds overhead to compile the whole project. The more a project has references, the bigger the overhead is. At work, we have about 100 projects, this become a real issue when the total overhead is 100 x [5~15 seconds].

Somehow, this overhead is more or less equal to:

overhead = [number of referenced projects] x [number of files in referenced project] x [complexity of the project settings (aka. property sheets)].

And as we saw in the table above, the biggest factor seems to be the number of files in the referenced projects.