So when you enter a character into a text field the affected part of the page is then reloaded with fresh HTML?

Liveview: it sends event to server and the server handle the event, then send "diff" to client (and automatically patch the dom) .. all of these via websocket, it's fast.

How does this degrade when connectivity is poor?

In my experience, not well (we had an important early user have to use our web app from a trailer using the wifi from the main house)

So might not be the best approach unless most of your users are at their desk