JSON encoding and decoding performance

[Clément Pit--Claudel]

Hi emacs-devel,

I'm running into performance issues with JSON encoding and decoding.  I have a 
language mode whose eldoc function makes a call to a language server running as 
a subprocess, and small lags tend to be visible when moving quickly around the 
buffer.  Profiling suggests that roughly 25 percent of the time is spent 
encoding and decoding JSON strings.

Would there be strong objections to supporting a C JSON library in addition to 
the current ELisp implementation?  This was suggested at one point in 
https://lists.gnu.org/archive/html/bug-gnu-emacs/2015-03/msg00770.html .  It 
would be similar in spirit to supporting libxml-parse-region along 
xml-parse-region, and it could provide nice speed boosts to json-heavy ELisp 
programs.

Cheers,
Clément.

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