Generally, what I have seen and noticed helping people out when they have this issue it comes down to your VRAM on your card being maxed out, or close to maxed out. This game eats up VRAM.
The general fixes are tweaking graphics settings. The two biggest performance hits are your draw distances and shadows. I have personally messed with both and seen significant increase in performance while maintaining visual quality. The textures on the finer details are the first things to start getting blurry.