You need to edit your diablo 3 config file. The location is for windows 7:
located in /Users/My Documents/Diablo III
Now there is alot of options, and i've tested them but do NOT change PreferencesVersion because you WILL CRASH.
With that being said here is the quick optimization: Scroll down thru your config file and find these 3 lines and make changes if necessary.
Now this one is "iffy" and you will have to choose what works best for your system. Personally I think it has to do with what version/directx its going to run. For instance 0 will make the game crash out and not load with a direct x number. I keep mine at 2. 1 seemed to speed it up, and 0 wont load.
Thats it!! I have a monster desktop with a radeon 6950 and a DV6 A8-3500m radeon 6620g laptop. My desktop screams thru the game but my laptop was jerking/stuttering all over the place.
I hope this helps someone. Please!!!! if either of these worked for you put down your VIDEO CARD MODEL and if it worked or not so we can help others.
Hopefully enough people see this and can squeeze enough performance out of diablo 3 to actually enjoy it. ALSO: If your system just seems laggy, dont fret turning down all the settings to LOW. To be honest you wont notice much but the shadows gone :)