If you have messed with the anti-aliasing options, that could be part of why you are lagging badly. Quote from the BIG help thread:
This is used for even out sharp edges on the screen, the higher the value, the smoother everything will look. This setting is very demanding and is not recommended to use for users with slower computers. Keep it on None unless you know it works well.
Also check your video mode, and other options such as water reflection. If you haven't already, you can try changing your rendering system to see which one works best for you.