As described here, CrossHair spends a lot of its execution time in a pure-python sys.settrace handler. Making this faster should speed up CrossHair quite a bit.

It would be great to benchmark how much would be saved by moving some of that logic to an extension module written in C; it wouldn't be possible to move all the logic, but many tracing invocations are no-ops, and avoiding those at least could help a lot. Cython might also be worth investigating.