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.
It’s becoming more common, but it mostly comes down to available tooling. At this point all three of the big game engines have a Vulkan backend available, but that’s a fairly recent development. And if a developer isn’t using a game engine, writing their own openGL renderer is easy, and writing a Vulkan renderer is a nightmare.
Also a lot of old proprietary game engines were written either specifically for DirectX or additionally for DirectX because in the olden times it was the most advanced and compatible rendering software.
Then, those developers move forward in time to work on other engines and focus primarily on DirectX because it’s still good, compatible, and it’s what they know best. OpenGL languished and it took a while for Vulkan to come out, catch up, and standardize their API.