The native Linux version doesn't have the new mod manager, but when I switched to Proton, the mod manager was just a black window and didn't work anyway. Once I skipped that, I also found that the Proton version had the same performance issue, so I think the game just isn't optimized well. Given the graphics, it should run better than it does. It's not a big complaint, though - the framerate is smooth and there were no other technical issues.