I have an i5-3550, GTX 970 and 24gigs of RAM with Windows 10. When I first installed the game I was really struggling to find graphics settings that would allow the game to run at least semi-smooth but ended up having strangely choppy performance pretty much regardless of what I tried even in the menus.
That is until I compared my settings with my friend's game - who had no problems running it whatsoever. The thing that finally made the game run smoothly was disabling true fullscreen and going with a borderless window. The difference is amazing even in the smoothness of the main menu screen which for some reason was around 20fps with true fullscreen before. No more lag now. All on Ultra (No SSAO, FXAA on) gets me around 40fps, but it's smooth and vastly superior experience to lower settings with 40-50 choppy fps.