River.com is the worst offender of web bloat I've seen in the bitcoin space.

Their site uses a framework called Hotwire. It uses websockets to receive HTML updates.

Doing simple actions like opening and closing a modal/popup will use more bandwidth than loading the entire React library.

River probably uses Hotwire for security reasons. But it makes the site frustrating to use on slow devices. Additionally, it means there is no API for brokerage accounts. So even power users have to use River's hot mess of a website.