Source is this video:
Windows Was The Problem All Along - Dave2D
We could obviously compare performance between windows and steamOS before on the steam deck, or between windows and Bazzite on other handhelds. But this is the first time we have had official windows and SteamOS builds for the same hardware.
I think it’s what Valve has branded their fork of Wine. It translates win32 calls to Linux ones, and DirectX to Vulkan. Probably some other stuff too idk
Proton is Wine plus DXVK and VKD3D, as well as a big pile of little tweaks and out of tree changes that Valve maintains to specifically maximize game compatibility and performance.