Shouldn't Linus Torvalds have run simple benchmarks like this, noticed the regressions, and done something about them before release? Or if not Linus, then.... somebody? I would still think that since Linus is in charge he would have some sort of regression-testing procedure in place, like OpenSUSE's OpenQA automated testing system. Or Phoronix. I just don't understand how the kernel can be so complex and everyone working on it so smart and then there's things like this and the power problems that no one notices and then requite Matthew Garrett to fix after he gets told it's impossible to fix.
Linus is way to busy to do it for everything in the full kernel. Ultimately this should be done by the subsystem maintainers before code was passed on to Linus, one would hope.