As long as they all finish their jobs in time, they shouldn't run any slower at all. They run, they complete, they wait for the next frame. If resources are constrained enough to shrink that wait to 0 or below, then yes, everything slows down, and somewhat less gracefully than with a more fair and more complex scheduling system. But it sounds like each program yields explicitly when it's ready for the upcoming frame, so if a program doesn't have much to do, its "share" of the time is free real estate.