egui of course maintains packed texture atlases of loaded textures. you don't need to upload that every frame.
at the end of the day,it comes down to empirical performance of the resulting app. all these retained mode ui $100 android phone are slow as balls. this immediate mode app is the only thing that is usable. I will take empirical results over hypotheticals
Frankly, I don't think the UI being slow has anything to do with the immediate or retainer mode choice. Especially because Compose is partially in immediate mode as well. I think many of the animations of most apps are still in immediate mode.
To me it is about thread prioritization, which is usually terrible in most apps, sys calls that are usually super slow, and easy libraries that need to do a lot of work to render.
For instance, Compose's Text element is a full HTML renderer. The text devs pass to it needs to be parsed before rendering. That doesn't need to be there for most apps, but it is. Things like that is where CPU cycles are going, not the UI mode choice. Compose's UI graph walk to figure out what changed for rendering is indexed, so it easily goes into the
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