When you drive a car for a while there's this white smoke that follow you behind the car, and it seems the longer you drive the worst the fps gets. The smoke stays for a very long time (See the pictures) and just moves with the wind. Even when you get out of the car your fps stays very low until (It seems) the smoke has gone away.
Drive a car on the road at half to full speed for a good 10 15 minutes. The smoke should start to build up after that
I usually have 60 to 80 fps while running around, 50 to 60 when I get the car for the first time and down to 7-10 when the smoke starts to build up. Even if the car isn't desyncing as the passager it still lags like crazy. Happens with every vehicles I used so far.