This thing uses CPU. I originally needed it to slow down a game that was running on a way too high framerate. Some older games just limited their speed based on your CPU, and did not implement any framerate limiting systems of their own. Thus on modern hardware, you can easily have a game that runs at 30x the intended speed. Very hard to play I can tell you.
By putting the game priority to 'low' in Task Manager, it only uses CPU when the CPU is idle. This allowed me to use a simple bit of Javascript to limit how fast the game runs. This is the script that I used.