retained mode frameworks still redraw the whole frame typically, unless they're doing some type of partial rendering - that still has a cost though, and all modern ui frameworks retained or not will suffer without a gpu.

in practice rendering a frame of imgui or whatever is so fast and efficient that it's not an issue. we're talking a few thousand triangles and typically a small handful of shaders & draw calls.

How fast are shaders on CPU? Shaders are garbage. I remember many years ago I tried Ubuntu in a VM and it decided to use llvmpipe for rendering. There was a 30-second lag between clicking on anything and seeing the result. Since then I hate GPU-based GUIs.