[rgbrgb]

SPAs reduce the latency of data fetches because each new resource is a few bytes of JSON rather than some kb of HTML.

[rcsorensen]

Of subsequent data fetches. Not the first page load. Using JSON fetches to update documents also gives you the ability to not throw away the DOM/CSS and maintain the execution context.

If you use the right technology, you can give a usable application out of the gate without waiting and use JSON data fetches for every subsequent request.