Both of those Screenshot where taken from Different ET Installations with Different Settings, FPS is not Effected in any Way
The Problem is, ET is not Very Well Designed for large Open Maps, and thus I Can't Do anything about it, Specially when i don't have the Map Source.
I Dont Even Have the Source for Fueldump Desert, but, I Fixed it by Rewriting the Terrain Shader (which was poorly Written), and manually Editing the Light maps in GIMP to Fix the Black Mountain Tops and also added the Missing Sound Script