Thanks for the replies. At least now I know its not bugged for me, but it's "normal". The 8700k is overclocked to 4.9, so overall the framerate isn't that bad..just not what I expected (and the GPU usage was low).
I playead with the in-game settings bit more and its quite ok now (although one core is still at 100% all the time). Just hope this behaviour will change in the next game 😉