Loading a large node body sequentially for better performance
It sounds like you've hit an architecture (DBMS) constraint. I'm not aware of a way to break up a node body into segments easily (I mean i'm sure you could use a list of text fields in drupal -- but ewww).

I'm also not sure of a clean way to make the paragraphs index-able for asynchronous javascript fetching, i believe you run into issues like:
https://stackoverflow.com/questions/19477494/per-paragraph-commenting-system

What I would recommend is installing an in-memory cache like Varnish, or Memcache and storing a copy of the full page for anonymous users ... or learning Authcache for more complex scenarios.