First off i'm running
Core 2 duo 3.0 GHz
4 GB of Ram
Nvidia GeForce 9500 GT 512mb
Started game was getting 20 FPS max, turned off grass and trees and shadows, went up to 25 FPS (rest on low)
I then turned on High settings from drop down menu, FPS dropped to 14....
Lowered Grass to 10% and tree's to 10%, turned Shadows off BOOM 45 FPS
Bugger me if i know why this happens but get them settings up and see what you get, turn things down as needed.