I'm in a similar situation. I recently built a new computer and downloaded the latest version of VOTV. Everything was fine in the tutorial, but when I launched the story mode, my RX 5700XT's FPS dropped from 75 to 30-50. Meanwhile, MangoHud showed the GPU was only at 30%. I'm using PortProton with a Proton-LG 10 for the game. Changing the RHI doesn't help.