What do you mean? For gaming it won't perform well at high resolutions in recent games. For just using as a desktop it's better than most laptops with Intel integrated graphics and can do multiple displays with MST.

Digital Foundry: https://www.youtube.com/watch?v=UZKBSf3aLf4

Generally, 7th-gen console era games and earlier can often play at 1080p60, 4k30, or 4k60 depending on the demands. Newer games make compromises. FSR and TSR (upscaling from lower internal resolutions) effectiveness depends on the game.

I only use it docked when I want to do some work in the desktop mode with a keyboard and mouse. I think of the dock more for using it in Desktop mode than something comparable to the Switch running connected to a TV. The one exception is Steam's game streaming functions - a docked Deck connected to a TV works great with controllers or keyboard/mouse connected streaming gameplay from your desktop in many games. Its a perfectly adequate little desktop PC in this configuration for a lot of tasks. The official dock even supports a dual monitor desktop config which is nice.

Gaming wise, the onboard GPU is pretty adequate for 720p (or the 800p of the native display), resolutions beyond this like the 1080p or greater found in most monitors are going to make a lot more games struggle.

720p is 921,600 pixels

1080p is 2,073,600 pixels (2.25x 720p)

1440p is 3,686,400 pixels (4x 720p)

If performance scaling was linear, and given that the Steam Deck targets 30fps (up to 60), any game running at 30fps is going to be dropping to roughly 13fps at 1080p at the same settings.