An easier way to speed up Project64 without upgrading your PC is to experiment with the Counter Factor values. Higher values will skip more frames per second (FPS), but the chances to make it faster is not much, plus this feature isn't effective with all games.
Another alternative for slower computers is to use 1964 instead of Project64, because 1964 has a frame skipping feature... or well, you can use a plugin that has frame skipping feature to work on Project64.