Performance Problems

I've been experiencing the whole package: lag, stutter and inconsistent framerates. The game rarely reaches 60 FPS, often it crawls around 20, sometimes even lower. The game freezes for 1-5 seconds on a regular basis. The longer I play the worse it gets, to a point where the game messed up my PC so hard that I basically have to restart the system or wait like 15 minutes for it to calm down. The game also regularly freezes when loading, always approx within the the first 10% of loading. I don't know if that is relevant or related to the problem. It's horribly frustrating because I really do enjoy this game, but as it is now it's borderline unplayable. I'm pretty sure that it's not my specs. I should be able to run this game just fine with them.

OS: Windows 7 Professional (64 Bit)
CPU: AMD FX-6300 Six-Core (3,5 GHz)
GPU: NVidia GeForce GTX 950 (2GB RAM)
RAM: 8 GB

I have asked around on the Steam forums and got the advice to force my GPU as the preferred GPU via the NVidia control panel. After I did that the performance became notably better, but it is still not very good. Is there anything else that can be done?