Hmm, good point, but xscreensaver runs at nice +10 by default. I think it's more of a problem with latency. My script spends 99% of its time asleep, so while it's sleeping the scheduler says, "OK, nobody's using any cpu. Even though xscreensaver is running at nice +10, we'll give it a chunk of cpu, because nobody else wants any." Then when my higher-priority process pops back up and demands some cpu time, there's a few hundred milliseconds of latency before its request is granted.