Proton with DV9K is - for a few games - faster than native windows. I don't fully understand it, but apparently the process of automatically rewriting shaders from DX to Vulcan can speed things up.

I think you mean DXVK.

It's pretty straightforward to have an abstraction for file systems with "special" file locations, it's been common in app sdks for at least a decade in a half. Unity doesn't have something like that?

Eg you would never use /tmp or %USER%/AppData, but call a function File::getPath(TempDirectory) or something like that.

It does, but I'm guessing it's just one simply understandable example.

C# has the very Windows-centric https://docs.microsoft.com/en-us/dotnet/api/system.environme...

Unity itself has the Application.*Path properties for some more general cross-platform-aware paths. https://docs.unity3d.com/ScriptReference/Application.html

Curious, why targeting DirectX, rather than Vulkan... it's not like you think about releasing on Xbox, do you ?

I remember writing some GPU instancing optimization that had #ifdef for DirectX 11+, but that code is old. Now I've learned how to go without it.

It's just all those small things that has to be right to make it work cross platform.