> web technologies for UI … slow, and heavy

It’s not the web platform features that are slow and heavy. It’s all the JavaScript on top of it. You can make a UI on the web with just HTML and a bit of CSS. Nothing beats that in terms of performance and small size*. But if you add several MBs of JavaScript frameworks on top of that, then it becomes what you described.

*Try the HTML version of Gmail in your browser https://mail.google.com/mail/u/0/h/. Just HTML and CSS. It’s the fastest email client in the world.

edit: I timed it. HTML Gmail loads and fully renders in about 1 second.