This is just plain false. Retained UIs like the DOM and what your OS uses only ever render when something changes, so the vast majority of the time they sit idle. There's extensive effort throughout the entire stack to do as little work as possible.

For instance, the mouse cursor is composited on the GPU during scanout. That means simply moving your cursor requires zero rendering.

Another example: When typing only the newly typed character and caret are rendered. The rest of your entire screen is reused.