bug#31138: Native json slower than json.el

[Eli Zaretskii]

> Cc: sebastien@chapu.is, yyoncho@gmail.com, 31138@debbugs.gnu.org
> From: Dmitry Gutov <dgutov@yandex.ru>
> Date: Mon, 22 Apr 2019 18:41:00 +0300
> 
> insert-file-contents probably uses code_convert_string somewhere inside, 
> right?

Yes, but that's irrelevant.  (You should have used
insert-file-contents-literally instead.)  And I don't think we will be
able to speed up the general-purpose decoding routines: no one said
the original file is encoded in UTF-8.