A way to increase drasticly your FPS is with the following tweak:
Go to the line
And increase the number, the more you increase, the less the density of grass will be generate, go to like 100 to 300
For an even better FPS change the two following line value :
This will remove winds on grass.
Oh yah and the ImingrassSize can also be reduce to INCREASE the density for those who have enough power to increase it.
Can also be changed to increase or decreased the distance you see grass, if you decrease the maximum grass, you could increase the distance without noticing any difference to your FPS and the game would overly look better since well, at least you wont see it appear at 2 feets in front of you
Also a very simple tweak for those who have mouse "lag", change
ps: These tweak are mad in the Oblivion.ini in your My Document/My Game/Oblivion, you should also put the file in "read-only" once you saved if you dont want to take the risk that the game change back all your tweak to default