I'm curious about caching potential with this approach.
Meaning: If the javascript (and other content) loaded via #render is highly cacheable, can this lead to pages that display as soon as the JSON is loaded?
I tested only on small websites, but the performance that I am experienced surprised me a bit. The render is very small and fast and as you said, like the JSON page (that it is also very small because there aren't any HTML tags), very cacheable. More testing is needed.
Meaning: If the javascript (and other content) loaded via #render is highly cacheable, can this lead to pages that display as soon as the JSON is loaded?