Dolphin tends to run slower on macs, and your graphics card dosen't help much either... Installing windows via boot camp would allow you to use the Directx 9 plugin which should give you quite a speed boost...
Checkout my latest project ( it is a program that downloads and compiles dolphin on mac and linux now): http://code.google.com/p/dolphin-emu-downloader (It's unofficial and maintained by me, not the dolphin dev team)