"The average CPU utilization on this graph isn’t much lower, at 11.77%, but the shape of the graph is distinctly different. The GTX 980 Ti’s frame rate is roughly double that of the R9 Fury X (we benchmarked with ambient occlusion disabled, since that mode causes continual rendering errors on the AMD platform), but the CPU utilization doesn’t keep spiking the way it does with the AMD cards."
"But there’s no evidence that this actually changed anything about how the game performs on AMD cards. Nvidia’s own website acknowledges that Gears of War uses HBAO+, but says nothing about PhysX. Given the age of this version of the Unreal 3 engine, it’s possible that this is a variable left over from when Ageia owned the PhysX API; Unreal 3 was the first game engine to feature Ageia support for hardware physics."
"If you’re an AMD gamer, my advice is to stay clear of Gears of War Ultimate Edition for the time being. There’s no clear evidence that PhysX is causing this problem, but the game runs unacceptably on Radeon hardware."
http://www.extremetech.com/gaming/224216-is-nvidias-physx-causing-amd-frame-rate-problems