while not a completely terrible idea, sleep doesn't give you any guarantee of when it'll wake your code up again, only that it'll be asleep for at least X time. I think this is ok, but someone with more info on these sorts of bugs would be better place to comment.
It also seems reasonable to add a random length of time to the sleep time, so that differences in computation time are indistinguisable from the randomness added by the random variable. I could be completely wrong though.
(also, you probably need to force with seq or deepseq the result of purecryptofunc before getting the second time, or you might be adding that evaluation time to another part of the code, and then this code becomes just a constant time added to the computation)
It also seems reasonable to add a random length of time to the sleep time, so that differences in computation time are indistinguisable from the randomness added by the random variable. I could be completely wrong though.
(also, you probably need to force with seq or deepseq the result of purecryptofunc before getting the second time, or you might be adding that evaluation time to another part of the code, and then this code becomes just a constant time added to the computation)