Portability means making sacrifices. I dont need 4k 200fps on a hand held on the bus, I just want to be able to jump in when ive got some down time and im not at my main machine.
I tried Skyrim on my Linux Desktop a while ago and it ran like ass. I couldn't push more than 45 FPS even without mods in 1080p with a 6650 XT. I know their engine is garbage with newer hardware but that seemed still ridiculous.
The deck runs at 800p maximum and has an FPS slider to cap the framerate so you can get more juice out of the battery.
Capping it to 40 FPS on Steam Deck would actually be an Ok thing to do if it means you can run the device longer while playing portably.
So under those standards you wont notice it running bad at all. Its funny how it works, but for some games the experience feels better on the Deck because of how good it is at scaling down everything. Also sometimes the lower native resolution makes things work smoother than they would have otherwise.
Honestly, nowadays Steam Deck compatibility is only meaningful for multiplayer titles. Most everything else works without many problems. Whether it's officially compatible or not.
It's a good signal on minimum hardware requirements and controller compatibility, but that's about it.