Yes, run https://github.com/antonblanchard/will-it-scale on operating systems with hilarious grandstanders like OpenBSD and Illumos, then run it on Linux and compare the results. Run a highly parallel make on a modern server and tell me which OS remains usable for other work. OpenBSD runs acceptably on 2-4 core laptops with HT disabled because a desktop workload doesn't typically invoke a lot of the kernel implementation during steady state, but you can do it there with a moderate disk and network load. That said, syscalls are more expensive in OpenBSD than contemporary operating systems so even as a uniprocessor OS it is funny.