I have codebases which scale almost linearly with cores. I built a 56-thread Xeon machine. One of my projects compiles in two minutes with single threaded make and two seconds with all 56 threads going.
Not the OP, but we have big projects written in C containing hundreds or thousands of source files and builds scale almost linearly, at least up to 96 cores (I tested on ARM servers).
You have to be careful about how you write Makefiles - not using Makefiles in subdirectories, but writing non-recursive Makefiles. And you have to sometimes divide up large source files.
It's usually a good idea to do a test build with make -j1 and pipe the output through moreutils “ts” utility, then sort by which operation takes longest and try to break it up / parallelize it.
