Zehmnas: I have a GTX 970 4gb with the new driver, an i5 4690k and 16gb RAM, that puts me between the minimum and recommended specs, a bit more to the recommended side. Bought the game on GOG.
But right now I'm having a really hard time with the game staying almost at all times below 30 FPS. I'm playing on fullscreen 1080p with low settings. Furthermore, there seems to be no difference in performance regardless of the graphic settings I choose (even ultra). I tried to restart the game after changing settings and tried to change settings on the main menu and there's no difference.
I'm doing okay'ish on medium with reduced shadows on an i7 6700k with 32 gb of RAM and a GTX 970. Not exactly super smooth, and I will probably have to tinker settings down to about low'ish to get the framerate where I want it, but pretty much playable, I think.
Do you have other stuff messing with your performance? Security suites, maybe something else hogging CPU?