There are a few benchmarks around the place of the various Java JSON implementations. Any version 0.2.X or earlier has very low performance compared to the fastest packages in the benchmark.
Current Performance (0.3.1+)
JVM Serializers Benchmark
One of the common benchmarks I have seen around the place is the JVM Serializers Benchmark. The original adaptors for JSONiJ are for the 0.2.X series (which is slower) and they use the JSONMarshaller for deserialization. IMHO is an unfair comparison to the other packages that a manually assembled from optimal use of their API's rather than auto-magic (and slow) Java reflection.
I have implemented a new updated 0.3.1 adaptor using a similar pattern to other JSON packages in this benchmark. The following is the resulting comparison;
The source of this should be available to run yourself but you should take it with a grain of salt. I have no idea if any of the other package developers were consulted to help with the adaptors so they may not reflect the package performance.