hash_max sets how much memory PyMOL uses when ray tracing. Use more then PyMOL will ray trace more quickly, less it's slower. For large scenes, it's useful to turn up this value. Simpler scenes probably don't need it. If you set hash_max too high, then PyMOL can (and will) crash if it can't secure the necessary memory.
set hash_max, int
where int is a positive integer. (I'm not actually sure what good boundaries are for the hash_max setting. Anyone?)
set hash_max, 200