HN is fast enough on an old 600MHz A8 processor from 8 years ago. In fact, multimedia aside, basically every static site that doesn't overdo the CSS will be fairly fast on fairly old processors.
Gmail or Facebook aren't the problem. People could be convinced to use desktop apps for them fairly easily (just like they do on their phones). The issue is all the other one-off apps. Users can't be bothered to go and download some random app, so the company runs that app in the browser. You simply cannot fight that level of market pressure.
Would you consider installing and running dozens of random untrusted 3rd party applications every day on your real OS even if they were (somewhat poorly) sandboxed? Because that's what the modern web is like.
I wouldn't mind opting-in to additional "interactive" features for webapps that I want to trust but having everybody (including 3rd parties of 3rd parties of the website I'm browsing) run code on my computer when I'm on some random webpage is just insane when you take a minute to think about it.