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.
You are forgetting backwards compatibility with ancient software that Windows still supports after 30+ years.
A lot of businesses need that in order to function.