Hello guys.

Firstly I want to underline, that this fix may not work for everyone. It worked for me so maybe it will help some of you guys.
Fix is VERY simple, all you need to do is change your Post Process Anti-Aliasing(in Anti-aliasing options) to Filmic SSMA 1X or SSMA T2X. It allowed me to play almost in constant 90fps and decreased start stutter to 1-3 seconds(while it used to be 10 or even more).
I know it sounds just to easy to bo true, but it helped me and helped my friend either. Before setting this I had FXAA turned on, the performance was much worse and I couldnt use shadows etc. I guess Raven ♥♥♥♥ed something up with FXAA and it's just bugged, so I strongly advise you not to use it. That would be it, you can try using other Post Process AA and check whether there are any differences between them or not.

My rig: i7 6700K, 16 GB 2400Mhz, Gtx 970 G1, game installed on HDD.
Cheers.