This is not new. ExtJS existed a decade ago. But the performance was so bad that we threw it out of the window at evaluation time and wrote our own very light

This is not new. ExtJS existed a decade ago. But the performance was so bad that we threw it out of the window at evaluation time and wrote our own very light weight augmented view components which provided exactly this class of functionality. For example, we render a bog standard table, then decorate it with some javascript which can collect a composite part of the table if there's a change of state. Basically a partial page refresh. The code is tiny for this, the first hit is the view that the user wants to see with no RTT, minimal script load time (we use jquery to attach event handlers and that's it). After that we just suck the changed part of the page body down. Fancy a 50ms RTT to a page hit? We do that.