Try these settings. Ex. you can from 25fps in low to 50 fps in high.
Open up options, video.
1) select High
2) select shader : 2.0 (some cards might have better result with 3.0, still try it.)
3) deactivate shadows
4) anisotropic filter quality : 4 or 5
5) ground : normal
6) grass : off
7) OPTIONAL : Force vsynch in your 3D Card software