Hi,
I've benchmarked serialization and parsing of JSON with and without explicit encoding. I've found that leaving out the coding makes both operations significantly faster – from a speedup of a factor of 1.11 ± 0.06 for parsing canada.json to 1.57 ± 0.08 for serializing twitter.json. Other speedups are in between, but the speedup is always significant (to at least one standard deviation). All unit tests pass when leaving out the coding steps – which isn't surprising given that currently the coding operations are expensive no-ops. Therefore I'd suggest to document the internal string encoding in lisp.h or character.h and remove the explicit coding in json.c and emacs-module.c. It's very unlikely that the internal string encoding will change frequently, and if so, the unit tests should catch potential issues caused by that.
Philipp