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’ll add for completeness that vkd3d-proton handles DX12 titles, and of course OGL and Vulkan are supported natively.