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.
DXVK (which also runs on windows) alone gives you a huge performance benefit. Playing world of warcraft on windows I’ll see about a 30% reduction in CPU usage and higher performance.
Proton doesn’t just get you to almost matching Windows’ performance. Proton easily outperforms windows even on higher end hardware where windows bloat isn’t a concern.
While proton enables that, that’s still just vulkan outperforming DirectX.
So technically proton isn’t improving performance here, it’s just allowing the game to run on better performing systems (like Linux and vulkan).