Next up, this patch:

No idea what this does, but it helps a lot:

%time ./test 9.000u 0.000s 0:09.11 99.2% 40+1324k 0+0io 0pf+0w

That's much better than the 280+ seconds from before. But it's still nearly twice as long as Linux takes.

There is more weirdness though. If I freshly boot the system I get timings like these, and even nbench reports decent scores. However, if I do a couple things like rerun/recompile nbench, then at some point something 'breaks' and the performance goes back down to what it used to be.