Really terrible performance for the last few patches,

I don't remember exactly when it started but after a patch a few months ago the game suddenly started running really badly. It would nearly freeze every so often start running at maybe 4 frames a second (I don't know how many frames it was actually rendering but it looked and felt like a slideshow.) I kept waiting for a future patch to fix it but nothing has gotten better since. It might even be getting worse.

Has anyone else experienced this? I've turned everything down to the lowest setting except resolution (1920x1080) and it doesn't seem to have helped. I'm using a kind of old PC: i5 4690 (3.5ghz) GTX 970 and 16 GB of RAM but it ran better when it was new...