According to various benchmarks, json-lib, based on the json.org implementation, is one of the slowest implementation for json parsing. Moreover, this project hasn't been updated since 2010-12-14 (latest release), and activity on the user mailing-list is empty.
I found jackson, which seems to be a lot faster, and activity is bigger, with a lot of releases (next to monthly).
Here are some links:
Update on State of Json-parsing Performance: http://www.cowtowncoder.com/blog/archives/2009/02/entry_204.html
Upgrade from org.json to Jackson, piece by piece, using jackson-module-org-json: http://www.cowtowncoder.com/blog/archives/2011/03/entry_447.html
project's documentation: http://wiki.fasterxml.com/JacksonDocumentation