Antoine, yeah, it's probably turbo boost related. There is no easy way to turn it off on mac os x, though. I hope Victor's patch to perf.py will help to mitigate this.
Victor, Marc-Andre,
Updated results of nano-bench (best of 10):
-m timeit -s "loops=tuple(range(100))" "sum([x * x * 1 for x in loops])"
2.7 8.5 3.5 10.1 3.6 8.91
-m timeit -s "loops=tuple(range(100))" "sum([x + x + 1 for x in loops])"
2.7 7.27 3.5 8.2 3.6 7.13
-m timeit -s "loops=tuple(range(100))" "sum([x - x - 1 for x in loops])"
2.7 7.01 3.5 8.1 3.6 6.95
Antoine, Serhiy, I'll upload a new patch soon. Probably Serhiy's idea of using a switch statement will make it slightly faster. I'll also add a fast path for integer division.