We are still a few months away from the official Steam Deck release, so the dev kit performance could actually be improved in the meantime. Still, demanding games like Shadow of the Tomb Raider and Cyberpunk 2077 appear to be hitting 30 fps even on high settings thanks to the included LPDDR5-5500 quad channel RAM.
https://www.notebookcheck.net/Steam-Deck-dev-kit-gets-tested-holds-its-own-in-demanding-games-with-a-bit-of-tweaking.567335.0.html