Almost 20 FPS more for me since i deactivated the mirror ! ( 1 Spit, over water, no other action/objects) As said by someone else, it helps a lot but when it was for 5 FPS for him, it's almost 20 for me ! I had noticed that my fps was downing a lot when i was looking ahead, and then noticed that it was downing even when there were only blue sky plus a small (even very small) part of the rear mirror...
In more, this "famous" rear mirror is totally buggy and crappy atm !
