>
> Thank you for your report and the data. Is it possible to have the
> file with which you've seen this problem?
>

Looks like the problem occurs only with accented characters present, so I created a json file with them. Attached.

Warning: Long lines!

After opening the file, just run M-x json-pretty-print-buffer  on it to see how long it takes.


If you run M-x json-pretty-print-buffer on the already pretty file then it's much faster,so maybe it's related to the single long line generated by json-encode.