Unplayable for me too, on a 4.6ghz 2600k with 16gb of ram and a 2gb 6970. Lowest settings at 1024×768 I’m getting 20fps where I used to get 40-50 at high 1920×1080. GPU useage in MSI afterburner is showing 99% constantly while in-game. CPU/GPU temps are fine.
I’m running windows 10 64bit but have been for a couple of weeks and it was fine prior to the patch. Also tried updating AMD drivers but made no difference.