Hi, I just thought I'd share my tip on what to do if D3 is in slow-motion and/or you have a low FPS on your Mac.
1.) Quit every application that is running in the background.
2.) Do NOT set your brightness to 100%. Decrease it to about 75% - This helped me the most. Ever since I decreased the brightness, I had no slow-motion nor did my CPU fans run at 4200 rpm - they are constantly at 1000 rpm which is normal.
3.) Turn everything low in the settings menu except the resolution (if you like it better with your display's native resolution - lol who doesn't.). I also have "Physics" on highest but you can turn that to the lowest setting available I guess.
4.) (optional) Also try to avoid putting your Mac to sleep since I read that waking it up from sleep might decrease the graphics performance - known issue on Macs.
Hope I could help and let me know how D3's doing after applying these.
PS.: I have a late 2009 iMac 27" with ATI Radeon HD 4850 512MB VRam