I’m all about some marketing hype, but it’s worth noting that these are just fresh takes on already solid (dare I say old) ideas.
Turbo (“The heart of Hotwire”) is an evolution of Turbolinks, which is a terrifically simple base idea: intercept clicks on internal links. Rather than the browser doing a full page refresh, fetch the contents of the new page, plop it in place, and History.pushState() the URL. Now you’ve got a Single Page App feel, but you didn’t have to build a SPA. That’s mighty convenient if you’ve already built your app in Rails with ERB templates.
But is that actually efficient? Well, it hasn’t been particularly popular so far. The thinking has been that the network is the bottleneck, so let’s send as little as possible over the network. “As little as possible” typically translates into JSON, typically. If you get JSON on the client, now you need a templating system on the client to turn that into usable DOM. With that technique, you’re paying two costs: 1) loading a client-side library 2) data-to-DOM processing. If you sent “HTML over the wire,” you pay neither of those costs (faster), but theoretically are sending beefier payloads across the network (slower), which assumes that HTML is heavier than JSON, which is… questionable.