I didn't yet have time to try the branch, but what you say begs a
 question: does the branch attempt to solve the problems of JSON files
 only, or does it attempt to solve a more general problem?

 
 There is nothing json-specific in the branch. All the js/json stuff is
 on master already.

That doesn't answer Eli's question, AFAIU. The branch demonstrates that,
with the Javascript/JSON improvements you installed recently on master,
editing the 20 MB dictionary.json file without using locked narrowing
around fontification-functions is reasonably efficient.