The grass is present in the mission cause the missionmaker decided the grass should be on. Edit those missions which you want to play to remove the grass. (that only works if you are hosting the game. if your friend hosts it he should be the one removing the grass from the mission, or you sent him the mission).
If you do know how to remove the grass missiosided search our forums (also in the armed assualt section) it was explained enough times and i am to busy with work atm.
About you lagging so bad,have you tried different drivers for your GPU? Both older and latest?
Close all background programs.
Are you hosting the game and playing on the same computer? If so thats not really adviced. Run a dedicated (or join someone elses) and see if your performance increases.