I agree with your final statement and the seeming lack of research, but are you sure your characterization of JUCE is accurate?

You’d surprise a lot of people if you were right about it using GPU acceleration for its UI framework. It does have an OpenGL container that fits into its view object hierarchy if you want to write your own accelerated component, but the rest of the UI is pretty much standard event-loop -> platform-API dispatch. It’s accelerated where the underlying platform-API is accelerated but not in any kind of explicit or fine-tuned way. The focus has always been more on portability than performance, even on the audio side.

(And of course the high-priority audio processing runs in an entirely segregated context from the UI, so the performance characteristics of the two pieces are decoupled anyway.)