Running the game with Proton seemed to run slighly better than the native version, so that's what I chose to stick with.

You get the option of using a DX11 renderer, or an "experimental" DX12 renderer. Since the DX12 one was labelled experimental, I mostly stuck to the DX11 one and had no real issues. I briefly played on my workstation with a 3440x1440 monitor as well. There, I did experience some performance issues with DX11. I switched over to DX12, and that seemed to improve the situation there.