Extreme Lag

I used to be able to run the game perfectly with a solid 60 fps but now it averages 1-15
It started to lag around the 0.5 update releases and I don't know why, I love the game but I just can't play it right now.

Pc Specs.

RAM: 16 GB
CPU: Intel i7-4790 3.60 GHz
GPU: Nvidia GTX 970