To be honest and brief, I have no idea what the problem is.
I do not deny that Dx12 may experience greater bugs compared to older Dx-s (see War Thunder trying to run under Proton), but this delta exists also with Vulkan-to-Vulkan.
If/when I can get an AMD PC with a GPU that’s got out the last 10 years I’ll test that one too, but any and all kinds of games just lose performance.
Some do due to DXVK (see the Reddit Link I posted above), some may do because of a lack of NTSYNC (the message you responded to), others I have no idea, because games such as Quake Remaster and “Doom 1 + 2, the Remaster” can render with Vulkan directly, but still lose from 100 to 300 FPS when running through Proton.