So today i figured out a solution to this problem.
first these are my specs:
Mac: Macbook Pro 2010 Model
Processor: 2.4 GHz Intel Core i5
Memory: 4GB 1067 MHz DDR3
Graphics: NVIDIA GeForce GT 300M 256 MB
Software: OS X Lion 10.7.4
Now, when you enter the game go to options, change all the in-game video/graphics setting to low, and turn off shadow and cluster. Once done turn down your resolution to as low as possible. For me i am using 1024x640 (widescreen). There you go, thats all there needs to fix this problem.
Before i did this my FPS (cmd + r) was 65-60 thats when i am in town, but as soon as i leave to fight monsters it dropped to 5-30. Once i reached Act II 'OMG' it was so terrible 2-10 FPS. But as i change the setting my FPS is now 20-40 when im fighting monsters or in a cave. Of course there will be a little unexpected lag but so far has not affected my gameplay.
When playing quite a long time on the macbook pro, it does become really slow, all the commands, i have no idea why, but my guess is something to do with the processor. But for me to fix this i just simply restart the computer and let it rest for a while.
P.S. Sorry if my english isn't great and contains any grammer mistake, english is my 2nd language.