This mod mod reduces the polygon count of grass. This will increase performance without a noticeable decrease in image quality. Note that there are different versions of this mod: one for regular Oblivion, one for SI, and the low-low polygon versions of
This mod reduces many of the grass meshes down from 20 triangles to 4 triangles. The effect this has depends on your computer, but for most it is a sizeable increase in FPS.