The new Yajl – Yet Another JSON Library is very fast.
yajl serialize: 0.180 deserialize: 0.182 total: 0.362
simplejson serialize: 0.840 deserialize: 0.490 total: 1.331
stdlib json serialize: 2.812 deserialize: 8.725 total: 11.537
You can compare the libraries yourself.
Update: UltraJSON is even faster.