Regarding the "joke on the web", there's plenty of HN threads about Flutter where you will see this sentiment, not only about performance, also about UX (e.g. text rendering, selection, scrolling...).

I'm not sure if the web render engine has gotten better since then, and am too lazy to look up the links rn, but threads should be easy to find using HN search.

Still seems like a common source language + GUI toolkit that targets the web platform and various native platforms (mainly Android, iOS, macOS, Windows, and desktop Linux of course) without significant overhead has not been achieved yet. And it's questionable whether it's possible, given the special requirements (and capabilities) of the web platform and the different native platform.

Try using any demo app from their user showcase on web.