Below you see what i did exactly (my post on steam in “”). With that done my performence is fine again as it was in the beta. I have played the 2 new maps adn befor i did this i was droping into the 40s even without a horde around and now im at stable capped 64 fps. Only wehn hordes show up i drope below 60 fps.
"Deinstall the game, make sure the folder is gone, then go to >>>
C:\Users\YourName\AppData\Roaming\Fatshark and delete the “Darktide” folder.
Now install the game again, dont touch the config file. When its installed, start it and do nothing else, just click play. Dont touch any setting and play a mission or 2 and tell me if your perfromence is better.
I did exactly this ^^ and now i have nearly stable capped 64 fps on the new “Barow” map. Befor i did all this, i could bearly keep 60 fps with drops to the 40 with the default settings.
I really hope you guys have similar results ;-)"
Like i said there, i think something went wrong during the update and some left over stuff from the beta or else made problem. I cant say, i have no clue, thats your job. I really hope that helps a lot of people 
Worth a shot though!