Not very much at runtime. The main problem with metal is that it's not really compatible with DX12 or vulkan. DX12 and Vulkan are very similar, metal is not. I'm sure the conversion isn't as 1-1 and you lose some performance by doing stuff esoterically.
Proton is not an emulator. Games running on Proton typically have slightly better GPU performance than native DX. It's mostly a compiler.
It's sort of like saying that C++ is inherently a performance hit over C because you have to do more translation. Well, no, the translation happens ahead of time, and the result of the translation might be better suited to run on the hardware. For example, C++ has semantics that allow optimizations that are impossible in C. Rust ALSO has semantics that allow optimizations that are impossible in C.
But, I'm being sloppy here. Proton is many tools, and DXVK is just one of them, WINE is another.
But, of the modern graphics APIs, Metal is the most unique. And it does have good reason - the M series chips do have some unique GPU hardware that allows them to do certain things faster.
It's just that those things aren't generally useful or automatic, WE have to set it up. But we can't do that if we use automatic translation.
