I have a very high end computer and graphic card but still I used to get a lot of FPS lag on certain areas of the game or when, for example, I played with CM wizards, suicidal Witch Doctors, or even with a full game.
I've tried everything posted on battle net forums but none of the solutions provided worked for me, even setting all the video settings to lowest possible didn't work.
So today I was messing with some configurations on the Video Settings and I discovered that one simple setting solved my problem.
Guess what? It's really simple and now I can play Diablo 3 with max graphic settings and get no FPS lag, no matter what kind of game I am in.
Here is what you should do:
§ Lower the Max Foreground FPS
You have to find what's the optimum level for your computer, so far I found that setting it to 70 is the best configuration in my case.
I really hope this helped you with your FPS lag problems, and if you need further help with this matter, let me know, I might be able to help you out