I pulled the code from http://www.bonto.ch/blog/2011/12/08/json-libraries-for-ios-comparison-updated/ and tested it on my local machine.
I was surprised by the results
You have not done anything wrong.
Things have changed in iOS 6. Apple have greatly improved the performance of NSJSONSerialization. When using iOS 6 and up, it is the fastest.
NSJSONSerialization
JSONKit has also not been updated in a year. It is falling behind.