What is Fast Terrain?

Fast Terrain switches your background environment from 2D to 3D. If your computer can handle the graphics for toggling Fast Terrain off, definitely keep it off, it looks a ton better.

If you're looking to increase your FPS a dramatic amount without much setback, I recommend setting most of your settings on medium and your shadows on low. I don't notice much of an increase in graphics quality from medium to high, but I do notice a significant drop in FPS.