While the Steam Deck won't perform any better when connected to its (sold separately) dock, it also won't perform any worse when used in handheld mode. This may disappoint some folks, as it results in docked performance having the same constraints as the handheld mode's specs.
In an interview with PC Gamer, Steam Deck designer Greg Coomer said Valve decided to prioritize the handheld mode since that’s the mode it believes will see the most use. This means that, despite being docked and played on a big screen, the 30fps minimum required for the Steam Deck’s 800p resolution will remain. So don’t go in expecting a 60fps baseline if you decide to dock your Steam Deck.