westfw:
I think compilation time is generally limited by sub-process creation time/overhead for the 50+ processes involved in the build process. (which raises some interesting optimization possibilities.)
That's true. Windows is well-known for having relatively long process start-up and tear-down times.
Windows 10 actually has a bug that is not present in Windows 8 and earlier in which process tear-down is not multithreaded inside the kernel, which defeats having multiple cores when you're spawning many processes at a time. See 24-core CPU and I can’t move my mouse | Random ASCII – tech blog of Bruce Dawson for an analysis of that particular problem.