[Speed] Any changes we want to make to perf.py?

Some people have brought up the idea of tweaking how perf.py drives the benchmarks. I personally wonder if we should go from a elapsed time measurement to # of executions in a set amount of time measurement to get a more stable number that's easier to measure and will make sense even as Python and computers get faster (I got this idea from Mozilla's Dromaeo benchmark suite: https://wiki.mozilla.org/Dromaeo).