bug#20154: 25.0.50; json-encode-string is too slow for large strings

[Eli Zaretskii]

> Date: Fri, 20 Mar 2015 20:42:14 +0200
> From: Dmitry Gutov <dgutov@yandex.ru>
> CC: 20154@debbugs.gnu.org
> 
> Making the string 10 times longer increases the runtime by ~5 here (0.1 
> -> 0.5). Another 10x increase in length makes it run 4.3 seconds.

So maybe writing this in C is the way to go.