Well, with Python it doesn't seem particularly fast. I put together a small test: http://svn.colorstudy.com/home/ianb/msgpack_json_comparison....

It uses some random JSON data I found. msgpack is more compact (87% of the JSON representation), though not dramatically so.

json encode: 5.54msec simplejson encode: 8.27msec msgpack encode: 11.4msec json decode: 16.4msec simplejson decode: 4.06msec msgpack decode: 2.84msec

I'm confused about why json is faster at encoding and simplejson is faster at decoding. simplejson is fastest when you combine encoding and decoding.