Hi, im having the same problem, but not so radical as you!
I had 70 fps before and right now im having 40, but i changed the settings in the graphic menu -->i set the antialiasing to 0, now im getting 60 fps... but the graphic is a bit worse of course :S
And the game is quite unstable, im getting a few crashes, but im only playing in timeattack mode... dont know what the heck they have done with this update, but im not sattisfied with this at all!!!