Hey everyone,
Seeing that many of you have trouble, you should try these settings. I went from 25fps in low to 50 fps in high.
Open up options, video.
1) Select High - Don't Question!
2) Select shader : 2.0
3) Deactivate shadows
4) Anisotropic filter quality : 4 or 5
5) Ground : normal
6) Herb : off
ie. Also disabling water reflection may help as well