I'm having the same issue. Went from easily 150-200+ FPS w/ maximum settings 4k Borderless Fullscreen, to 40FPS minimum settings 1920x1080. Only change has been the new patch.

I have also noticed that my CPU usage went from sub 10% (spiking to about 12%-18% for brief moments, but never sitting at 15%+) to constant 22%-23% with long moments of 34%+. My memory usage also went from constantly 900mb-990mb (Never over 1,000mb) to starting at 1,098mb and slowly increasing to 1,700mb+ (Closed game out instead of allowing it to constantly attack my CPU's temperature.)

To me, the fact that the memory usage is constantly rising, no matter what zone I'm in or where I'm at kind of hints at a memory leak. However, even if it's not the drop in my FPS to be unable to hold 60fps at minimum settings is a clear indicator something is wrong. I already tried to tweak my Nvidia and Windows 10 settings, as well as some other "fixes" with similar issues others had in the past. However, nothing has helped.

My specs:
Motherboard: Asus Z-170A
CPU: Intel i7-6700k
GPU: GeForce GTX1080
RAM: RipJaw SkillSeries 2x8GB (16GB) 3200