It really can't be fixed: that machine is waaaaaaay too slow to do MP3s. It's more the machine's architecture than processor speed, but there's still no way to fix that thing except by upgrading its logic board with one from a 6360 or better.

All of the lovely technical details are here. It should be obvious that playback of MP3s, or any compressed format, will be unbelievably slow on that machine simply because of the layout of its logic board.