It's the graphics processing, i can run anything at full speed on my brother crappy athlon64 3200+ at 2ghz with the highest settings on a Ati HD 4850.In addition, what is the main bottleneck of emulator speed right now? Is it the cpu or graphics processing?
In my pc Athlon64 3200+ at 2ghz with the integrated Nvidia 6100 it runs slow with the lowest settings.
Haven't tested the emulator with a very fast cpu, but i suspect it will only gain a minimal speed boost on a fast cpu with a crappy graphic card. You will get better results with a good graphic card instead.
Edit: BTW, this post is based on nullDC, other emulators will benefit from a fast cpu instead of a good graphic card (M.A.M.E, No$GBA, etc...). Not all emulators are the same.