With imGui you must fully describe your interface at each frame. Because it's fully GPU accelerated it does not cost that much and run very fluently at 60 FPS.
But it drains your battery like crazy. Immediate mode GUIs are good for games, which already render the whole scene @60FPS and are expected to be costly, but re-rendering your whole window every frame even when idling is just a waste of power.
For non-game (desktopey) app, well, there's no reason to render at 60 FPS you should only render on user interaction otherwise go idle most of the time....