It's surely nothing to do with the grass, which is just a bunch of polygons. If you already used the textures resizer, there's not much else you can do, other than just reducing the Scenery Complexity slider.
In any case, there's no separate .BGL for the grass. You can have an idea how the grass is impacting performances, because there's an optimization that automatically switches it off over a certain altitude (about 200 mt from ground). However, it's just specific to grass because, even 3d taxiway signs will also switch off at that altitude so, it would be difficult to estimate how much are impacting each one (although, it's surely more the 3d taxi signs)
