The key part there is no EXTRA cost. Most immediate mode GUI libraries redraw 60 frames everytime, so there is a constant somewhat high cost, especially compared with a retained mode program.
To clarify, egui has a mode where it only redraws while you're interacting with the app (e.g hovering, clicking). It doesn't do anything while idle (e.g. In between keystrokes)
The key part there is no EXTRA cost. Most immediate mode GUI libraries redraw 60 frames everytime, so there is a constant somewhat high cost, especially compared with a retained mode program.