You say this with zero proof. Checking 100s of widgets to see if they overlap, updating their damage boxes, drawing the parts of the ones that got overlapped, computing clip bounds for each one, switching graphics contexts, to do all that adds up to "killing the battery"

Retained APIs are like using a binary tree where an Immediate mode GUI is like using a vector. CS principles say the tree should be faster as insert and delete are in O(1) but in reality, cache misses and similar things kill all the perf you supposedly gained by over engineering the solution.

The same is often true of retained mode GUIs vs immediate, especially with all the transparency effects in modern UIs. Computing the minimal amount of parts can requires a ton more CPU than just drawing.

As far as caching = retained, no. The difference between an Immediate Mode and a retained mode API is if you, the user, have to create and maintain a tree of retained API widgets. No one is going to implement an immediate mode API and try to have a text widget that word wraps and expect it to have to compute all the word wrapping every frame.