Once you set Shadows to very high in the Options Menu the game slows down. You have to use DXVK to run the game, version doesnt matter.
I already reported this last year when I was still using my Radeon HD 7970:
The game takes a nose dive from 134 to 29 fps once you switch the settings.
I tried today with my new Vega 56 and ACO:
with LLVM: 72,5 fps
with ACO: 206,3 fps
So we finally have the proof that LLVM is causing this. This might be the same problem we have with the long standing and still valid bug: