The fact that something doesn’t scale past X cores doesn’t mean that it is I/O bound! For most C++ toolchains, any given translation unit can only be compiled on a single core. So if you have a big project, but there’s a few files that alone take 1+ minute to compile, the entire compilation can’t possibly take any less than 1 minute even if you had infinite cores. That’s not even getting into linking, which is also usually at least partially if not totally a serial process. See also https://en.m.wikipedia.org/wiki/Amdahl%27s_law