Check out ultrajson (aka ujson) - (also see ujson4c[1] in the same org)<p>as far as serializing, ujson is 7x faster than simplejson in the <i>worst</i> cases I've seen (disclaimer: using the Python bindings.)<p>[1] <a href="https://github.com/esnme/ultrajson" rel="nofollow">https://github.com/esnme/ultrajson</a><p>[2] <a href="https://github.com/esnme/ujson4c" rel="nofollow">https://github.com/esnme/ujson4c</a> - "A more user friendly layer for decoding JSON in C/C++ based on [..] UltraJSON"<p>n.b.: There also exists a class of "simplified" JSON serializers. They implement <i>most</i> of the JSON specification but leave out some features (or restrictions) that make it more expensive. I don't know anything about these, perhaps another commenter can chime in.
If you need "fastest", are you really sure you need "JSON"? JSON has a lot to recommend it, but there are certainly <i>faster</i> alternatives.