> ... people scroll almost constantly in which case the ImGUI wins by a landslide
I think this is a misrepresentation of how fast scrolling is usually implemented.
For fast scrolling, you render the page (which is larger than the viewport = "what fits on the monitor") ONCE onto a GPU texture, and then all scolling happens on the GPU side (the CPU just tells the GPU the offsets into that texture).
Immediate mode has to recreate the texture every frame, instead of once for multiple frames. So "It might use more CPU" is quite certainly true.
I think you're conflating the IMGUI versus RMGUI question with whether rendering is done on the CPU or GPU. The two questions are independent of one another.
I don't think so: Full immediate mode rendering on the GPU would require scene graph and application logic on the GPU.
People don't do that stuff in shaders or CUDA, because they were neither designed for it, nor is it fast, nor is it pleasant to code.