[[But if you don't use the features, you should not need to pay the price!
If I understand correctly, the slowdown has been seen in repeatable benchmarks that can be run on both old and new kernel versions. Therefore the benchmarked code certainly isn't using any new features, but it still gets slowed down. Not justifiable.]]
Linus referred to the icache footprint(size) of the kernel, if you add features, even when not used they increased the size of the generated code so they reduce the performance.
Sure if you have an option to remove the code from the kernel at compilation time, then this issue shouldn't happen.. So which configuration did Intel benchmark?
Without specific figures, it's difficult to know where the issue is, I wouldn't be surprised that SELinux or virtualisation are the culprit: these features seems quite invasive..