Well, X-Ray is a feature in the Prime app that I like and use quite often and is very snappy. Since it's part of the UI overlay, I think it's handled by the WASM module (probably with local caching)
and not by the dedicated video-decoding hardware in the TV.
I agree that the browsing UI can sometimes take an extra second or two to refresh which I don't find to be a dealbreaker since it's the video/audio playback that I care about the most.