Ive been through all kinds of solutions for the FPS drop during gameplay. None of them worked until I found the following solution. This is happening to many players as we can see. This issue only involves multiplayer, especially when 4 players encounter a lot of mobs.
Blizz has no solution but I give it a good try:
Head over to this location:
C:\Documents and Settings\username\Documents\Diablo III
Open up D3Prefs.txt in a text editor and modify the following highlighted options to match the image.
This SOLVED THE MAIN PROBLEM for ME! I won't go through all other possible tweaks - as there are too many (which dont work ;-)).
Before changing this I had FPs DROPs DOWN TO 15-20, now I DO NOT DROP BELOW 50FPS! When shit hits the fan with crazyness going on I might drop to 40ish. If nothing crazy is going I play at a constant 80-100 + - FPS during 4 player and casting.
Asus GTX 780 ti
i7 4790 @3.6GHz
D3 on SSD
BenQ XL2420z Screen @120Hz
V-Sync OFF, other settings dont really matter I suppose. I did limit the FPS in fore- and background to 120/50. Rest on max quality.
No NVidia tweaks. All on performance - as your PC settings dont really matter - the problem lies within the corrupted D3 settings/programings.
Try this and let me know please! GL