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!!!
I just finished playing 3 hours online. 4 different tracks and no crashes or new problems to report. However I still have the same slowdowns with my fps as before, usually always approaching turns on many tracks and the stuttering increases after 3 or 4 laps. Game stars to stutter and slow down.
I tried iRacing later on and noticed the FPS was much lower there also. What I did was restored the factory settings in AMD Catalyst Control Center and It seems to have mostly fixed the problem! (Might be a little lower still, but got most my FPS back!)