Was the heavy "Crowd Performance Bug" mentioned here?
Setting "Crowd" in the graphics settings to anything alse than OFF results in massive prformance problems on NVIDIA cards.
The only way to maintain solid 100fps on an 1080ti is to turn crowd OFF. Thats okey, until you hit an invisible pedestrian and get a time penalty.
Nvidia has already taken note of this issue and is able to reproduce it, but its unclear if its a problem of the driver or engine...
Further information: https://forums.geforce.com/default/topic/1101511/geforce-drivers/dirt-rally-2-0-crowd-issues-with-nvidia/