Yea, what is this BS Linux kernel build process that is a known metric that everyone can test against? I compile projects dozens of times a day at my work and never once I compiled the linux kernel...
For what it's worth, I migrated from a consumer-grade i7 CPU to a workstation-grade Xeon and the differences in compile speeds in my everyday projects (less than 400 files) are still significant and very noticeably in favour of the Xeon.
Linux kernel compilation is just a benchmark that multiplies such differences between CPUs. Thus the buyers can -- hopefully -- make a more informed decision.
It also has tooling for "watching" test directories and auto re-running tests when certain files are changed -- which utilises incremental compilation under the hood and there indeed there is no visible compilation performance difference.