In a recent python project where I was sending multiple messages per second of data over a basic socket, I had initially just grabbed the cPickle module to get the prototype proof-of-concept functioning properly. cPickle is awesome for easily serializing more complex python objects like custom classes, even though in my case I am only sending basic types.
My messages were dicts with some nested dicts, lists, floats, and string values. Roughly 500-1000 bytes. cPickle was doing just fine, but there came a point where I wanted to investigate the areas that could be tightened up. The first thing I realized was that I had forgotten to encode cPickle in the binary format (the default is ascii). That saved me quite a bit of time. But then I casually searched online to see if any json options might be better since my data is pretty primitive anyways.
I found UltraJSON, which is a pure C json parsing library for python, and ran some tests. There are benchmarks on the project page for ujson, as well as other articles on the internet, but I just wanted to post up my own results using a mixed type data container. ujson came out extremely fast (faster than cPickle in binary)