When I'm running in any town, or no action on screen I've got stable 300 fps. When there are a lot of action on screen my fps drops to 20-30. I opened GPU-Z and noticed, that my GPU load is always at 98% but when it comes to fps drop gpu usage drops to 15% too. This problem occurs only in Diablo 3, no other games.
My pc specs:
i7-5820k @ 4.4 ghz
ASUS Rampage V Extreme
32 GB DDR4-3000 mhz
MSI GeForce GTX 980 Ti
Samsung 950 pro 512 gb SSD
Playing in Full HD (1920x1080) resolution
Also I tried to run game in 4k (DSR mode) and it runs perfectly, no fps drop and also no GPU usage drop.