Why not? If the raw renderer has unoptimsied denoise algorithm, what has that got to do with egui?

They seem to be edge cases of building extremely complicated renderers (that handle varied user files/scenes) in a very short time. The projects are weeks old. The original app's renderers were built and optimised over decades against user files. I would also assume the central renderer is a 3d surface, not egui.

Generally to the contrary, I was surprised how snappy their UIs feel in normal usage compared to the apps they're replacing.