Here's my suggestion: Force anisotropic filtering to 8x in your graphics driver control panel. Simple as that. Plus the textures will look better too. I didn't check 2x,4x or 16x anisotropic filtering settings, but it's possible as long as you force one of these, it might have the same result.
When I did this, I still had a very minor bit of lag in the woods at the beginning when the zombies chase you over the fence, but it wasn't as bad as before. For the rest of the game it didn't lag or get choppy once where it previously did. Let me know if this works for you.