I've worked on an app with an IMGUI on canvas. It was amazingly fast and responsive. Nothing which was built after that to try and replace it was anywhere near as performant.
Given how IMGUI is a bunch of branches with code that redraws the same pixels all the time, 60x per second, I'm still surprised this is considered very (if not most) efficient UI. It would imply retained-mode UI frameworks are strongly undershooting their theoretically possible performance.