I have just upgraded from my old Core i5 3570K clocked to 4.2GHz to to the new Core i5 8600K clocked to 4.7GHz. I have Geforece GTX 1070 and 16GB RAM clocked to 3000. Playing at 1920x1080.
The difference in performance is INSANE. Not only the framerate jumped up by 20 FPS on average, but I was able to move all the sliders to ULTRA and maintain about 60 FPS even in heavy combat.
Most notable, everything is far smoother and responsive, and the microstutter on fast driving or running is gone. It’s obvious that the throughput of the old CPU was already grossly insufficient.
I pass it as a practical tip to were the invested bucks make the loudest bang - I doubt that GPU upgrade would bring as decisive improvement as the game is clearly heavily CPU bound.