Citation needed. Whether the server is serving HTML or JSON it still needs to serialise the data, so I don't think serialising JSON is going to be significantly faster than serialising HTML. Plus then the client needs to deserialise that JSON before it can render HTML, so all of that work related to (de)serialising JSON is work which doesn't even need to happen if the server is rendering HTML. Not to mention the work on the client to parse and evaluate the JS which needs to happen before it can even start rendering HTML.

As for data across the wire, GZIP is a thing so again I would want to see real world performance numbers to back your claims.