bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Cc: yyoncho@gmail.com, sebastien@chapu.is, 31138@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 22 Apr 2019 16:54:19 +0300
> 
> On 22.04.2019 10:16, Eli Zaretskii wrote:
> > Please don't forget that json-parse-string first_encodes_  the input
> > string in UTF-8, then passes it to libjansson, then_decodes_  each
> > string element it receives back.  So we do some non-trivial processing
> > besides make_specified_string, and removing decoding only saves part
> > of that.
> 
> That actually means that speeding up code_convert_string might 
> *potentially* (e.g. if it's infinitely fast) yield more than 50% 
> performance improvement.

code_convert_string doesn't do much, it just calls the encoding or
decoding function.