Using ULTRA you may be using more than 3GB of VRAM and have plummeting FPS.
You should be capable of about 40 to 50FPS at 1080p with mostly max settings with that CPU and GPU.
I would check Unigine Valley or some other mostly GPU benchmark to see if there is some hardware or drive issue.
I would also monitor your VRAM usage and GPU frequency to see if VRAM is used up or if the GPU is being throttled low for some reason. (should be over 1700MHz likely on the GPU).
I have change the settings in game adjusting shadow quality, ambient occlusion, anti-aliasing and I got 60fps now ..
when I change the setting to ULTRA high i only get 20-30fps ..
no idea why before it is 4-8fps could be bug