Remember those awesome power controllers for NES that had the extra buttons? There was the sweet turbo button that spammed A or B so you didn't have to. One controller had a "slow" mode, which spammed the start button, which caused some games to enter/exit pause quickly enough to have the effect of slowing the game down. This is that for unix processes.
$ slowdown PID [percent]
There's also a speed_test program included so you can see the effects more easily and play with it.