Currently, all the Erlang based JSON encoders and decoders are very slow, and decoding and encoding JSON is something that we do basically everywhere.
Via IRC, it recently discussed about adding a JSON NIF encoder/decoder. Damien also started a thread at the development mailing list about adding NIFs to trunk.
The patch/branch at  adds such a JSON encoder/decoder. It is based on Paul Davis' eep0018 project . Damien made some modifications  to it mostly to add support for big numbers (Paul's eep0018 limits the precision to 32/64 bits) and a few optimizations. I made a few corrections and minor enhancements on top of Damien's fork as well . Finally Benoît identified some missing capabilities compared to mochijson2 (on encoding, allow atoms as strings and strings as object properties).
Also, the version added in the patch at  uses mochijson2 when the C NIF is not loaded. Autotools configuration was adapted to compile the NIF only when we're using an OTP release >= R13B04 (R13B03 NIF API is too limited and suffered many changes compared to R13B04 and R14) - therefore it should work on any OTP release > R13B at least.
I successfully tested this on R13B03, R13B04 and R14B02 in an Ubuntu environment.
I'm not sure if it builds at all on Windows - would appreciate if someone could verify it.
Also, I'm far from being good with the autotools, so I probably missed something important or I'm doing something in a not very standard way.
This NIF encoder/decoder is about one order of magnitude faster compared to mochijson2 and other Erlang-only solutions such as jsx. A read and writes test with relaximation shows this has a very positive impact, specially on reads (the EJSON encoding is more expensive than JSON decoding) - http://graphs.mikeal.couchone.com/#/graph/698bf36b6c64dbd19aa2bef634052381
@Paul, since this is based on your eep0018 effort, do you think any other missing files should be added (README, etap tests, etc)? Also, should we put somewhere a note this is based on your project?
 - https://github.com/fdmanana/couchdb/compare/json_nif
 - https://github.com/davisp/eep0018
 - https://github.com/Damienkatz/eep0018/commits/master
 - https://github.com/fdmanana/eep0018/commits/final_damien