What a run of a fast simulation did. Not a rate. It used to return cycles per millisecond, with None meaning both "nothing to do" and "finished" - so a caller could not tell those apart, and the one caller that used the number divided by it to guess how long the rest would take. Guessing elapsed time from work done is what breaks when a machine sleeps mid-run: the rate collapses towards zero and instant work is predicted to take minutes. This says what happened and nothing more.