This doesn't actually succeed (yet?) at avoiding blocking the UI thread while parsing large chunks of JSON. The async API means that browsers could parse JSON from fetch responses on a different thread, but it doesn't mean that they do.

Here's a jsbin experiment that shows that the main thread is blocked while parsing JSON using the author's technique (tested in Chrome 44 and Firefox 41):

http://jsbin.com/yutahe/8/edit?js,output