Well, in my case I have access to a couple servers with >250 threads, and it will happily peg all those out until it has to put vmlinux together, then it goes d

Well, in my case I have access to a couple servers with >250 threads, and it will happily peg all those out until it has to put vmlinux together, then it goes down to a thread or two for a bit, and then they all go back to 100% while the modules are processed. So, yes, but presumably given enough threads the limit becomes how fast one can collect and link the vmlinux which is mostly still done serially ala Amdahl's law. Oh and there is some IO/etc bounding at the beginning checking the config/etc. Some sites have kernel compile benchmarks and for the most part they tend to indicate nearly linear scalability. Although there tend to be two major config differences, some people benchmark with something like an allmodconfig vs people who benchmark with a more minimal kernel module setup. The former are the ones that can be as slow as an hour or so on machines without a lot of cores, and the latter are the ones that finish in sub 60 seconds on fairly modest hardware. But given enough cores its possible to turn the former into he latter.