Setting priority to high will not make a game that is written by a halfway competent developer, speed up...
It sounds like you are talking about a program called "GEAR".
I'm not sure how it works exactly, but as I understand it:
Gear hacks some functions in windows that are commonly used by game programmers to limit fps, create timers, etc... It uses hooks to run its own code rather than the native functions. Now it can do things like, remove fps limits (making the game speed waaay up) or add additional delays to slow it down.
