Hi, thanks for sharing this issue.

This is, categorically, a platform stability issue. VBCSCompiler is a shared process that gets used by all MSBuild runs on the machine to perform compilation as a service. As such, there's usually one instance of it that takes responsibility for all compiler runs that happen on the machine, and it hangs around for a while to pick up any new runs.

Apparently it's possible to disable this behaviour by setting UseSharedCompilation=false, which can be done in a project file or an environment variable, or in theory in your NCrunch custom build properties setting. In my experience, there is a performance cost for doing this. It might be worth doing it for your VS builds (which tend to be less frequent than NCrunch), since this might prevent the builds from interacting with each other.

Unfortunately I do not have the capability to troubleshoot the internals of this process. I suspect that this problem is specific to your solution or environment, as if it were not, we'd be seeing widespread reports of it. The first thing I would check is your analyzers, which may be getting loaded by Nuget package references. NCrunch itself does not execute any of its own code within VBCSCompiler.exe, but custom analyzers generally run within it. It may be that you have an analyzer that does not support concurrent builds.

Changing your CPU settings under NCrunch might mitigate this issue but are unlikely to resolve it.

Looking at the post you've written, it seems you've identified the GC as a possible source of the hanging? If so, it might be worth seeing if you can disable concurrent garbage collection for the VBCSCompiler.exe process. I don't have experience with making such a change for this process specifically, but I have experienced GC instability in the past and found it to be particularly infuriating and difficult to work around using other methods. It's also very hard to get MS to resolve it as it tends to be very situational and is hard to reproduce.