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.
The branch is not just for JSON. And it can demonstrate different things. People should try it and follow the original instructions, with different file types and sizes.