[2020] Sudden FPS drop on High End PC.

I'd find the latest graphics driver and do a clean install of it. Windows updates have a tendency to mess with graphic drivers, so a clean install is a pretty easy way to see if that's playing a role here.

The RTX 2080ti should be able to handle the game perfectly fine without needing the -force-d3d9 launch option - that's typically only needed for lower end graphics cards.