Have a look at: https://support.fatshark.se/hc/en-us/articles/360022719993--PC-How-to-Improve-Your-Performance-in-Vermintide-2

Otherwise please include this information in our dedicated performance complaints post:

There is nothing in the logs that would explain a substantial drop in FPS. If every solution in the article linked above has been unsuccessful I can only suspect either a deeper rooted issue that is likely to be outside of our area of expertise, or simply poor optimisation on our end.