I have found that even 2D games (eg Hades) aren’t reasonable to run at 4K, but basic gui desktop with a terminal emulator works fine.
3840x2160 = About eight million pixels
People underestimate the performance cost of 4K. The step from 720p to 1080p was a mere doubling of pixel count, but going to 4K was another 4x increase.
The choice of a relatively low screen resolution is one of the main reasons that the deck can run modern games so well.