Valve’s main goal is for their distribution to run on everything, performance is a low priority.

Are you trolling or something? This is categorically false, Gabe Newell himself expressed that with the Steam Deck (and therefore SteamOS, which was designed specifically for it in this modern version) their focus absolutely was performance as well as user experience. It had to so they could squeeze every last frame out of the AMD SoC. They literally developed their own micro-compositor (Gamescope) instead of utilizing e.g. Plasma (despite it being pre-installed for Desktop mode) for higher performance and quicker feature availability. Not to mention Nvidia GPUs still not being fully supported by SteamOS despite it being technically possible if they wanted to. Your claim about SteamOS' priorities is nonsense.

the words you’re looking for are “scheduler optimised for gaming at the cost of performance in other domains like power consumption”

So a better scheduler for gaming distros, which indeed was the context. Though I'd argue there are schedulers who're even objectively better for general desktop and gaming use as the one that treats every process equally isn't optimal to make either a GUI feel snappy or prevent microstuttering in games, but rather optimised for server workloads.

Gamemode is cool, however if you want to build your own Gabecube with some random distro it's absolutely not guaranteed to run during a Gamescope session or even at all, not all distros ship or enable it by default or you have to put "gamemoderun %command%" into every game's start parameters. Not to mention most users most likely neither know what that is nor do they want to, they want to play games and just use the computer.