I think a lot of this is running googles site compression tools over markup to get the minified .js compatible version for efficient download. If you can get the significant, page-draw reflecting events into the first packet of data after TLS, you can start to render.

I used to hate sites which inlined images. Then I realized, it meant all the icon draws for graphical elements on-page were in a blob of base64 in that initial download and avoided web cache lookups on icons a gazillion times.