They are not plenty fast. You know you don't have a fast platform when the best you can do is scroll + a couple animations at the same time.

And even then, HTML+JS apps often cannot do that without lag. Not saying that native apps are necessarily better (especially on Android), I die a little inside each time I unlock my phone and the unlock screen fails to keep up with my finger.

We had smooth scrolling in the late eighties on the Amiga 2000 (7 Mhz and 1MB of RAM) with CygnusEd: https://www.youtube.com/watch?v=L41oIvre9K0

The fact that we barely manage to do that with HTML+JS on what amounts to supercomputers (relative to the time) should not make anyone proud.

If your plan for speeding up a slow user HTML/JS interface is to rewrite parts using WebGL, you're severely underestimating the amount of support code you'd need in order to make that happen. Coming from somebody who has developed and maintains OpenGL user interfaces, you're better off just writing the native app.