from FAQ:
Q: Whoa, Dolphin is running as fast as a pregnant snail, what am I doing wrong?
A: The Nintendo Gamecube is a very advanced platform and (todays) computers are not fast enough yet to emulate it with the techniques we use. So... you aren't doing anything wrong! Upgrading a CPU or GPU won't gain you much more performance (you will only gain 1-5 fps). IF you decide to upgrade just besure to get a 64-bit CPU and a GPU which supports (at least) 2.0 pixelshaders.