Disclaimer: I'm the author of TouchJSON - the library the authors are claiming 100x speed up over.
As pointed out by other commenters the 100x speed up claim is incorrect. I added NXJSON to Sam Soffe's iOS benchmark test app and definitely did not see a 100x speed up.
I'm _guessing_ that the JSON deserialization actually failed when testing and the 100x speed up was due to the tests bailing out early. Sam'e JSON benchmark tool while handy isn't 100% fool proof - there's no validating the output of the code to make sure it matches the expect results.
While NXJSON is faster than a lot of it's competitors it really doesn't seem to do anything interesting. It is Obj-C based (you can definitely get better performance with straight to the metal C) and gets some speed improvements via macros rather than Obj-C message dispatches.
Unfortunately the 100x claims are floating around on twitter and are repeated here on hacker news. If it's too good to be true - it probably is.