It’s not like they built a new kernel specifically for the steam deck.
I agree that the majority of the impact being seen is from various components that aren’t SteamOS specific, however Valve does actually have a custom kernel for the Steam Deck “linux-neptune” (there are quite a few mirrors for browsing, but this is the official source).
I believe most of their changes are just to drive the deck’s hardware. Every now and then there are some changes that Valve contributes that lands there first before it gets upstreamed, for example the Arch Wiki calls out the Steam Deck’s kernel as a way to fix issues between HDR & VRR (shouldn’t be needed anymore on modern mainline kernels).
I agree that the majority of the impact being seen is from various components that aren’t SteamOS specific, however Valve does actually have a custom kernel for the Steam Deck “linux-neptune” (there are quite a few mirrors for browsing, but this is the official source).
I believe most of their changes are just to drive the deck’s hardware. Every now and then there are some changes that Valve contributes that lands there first before it gets upstreamed, for example the Arch Wiki calls out the Steam Deck’s kernel as a way to fix issues between HDR & VRR (shouldn’t be needed anymore on modern mainline kernels).