I have been playing a little with MsgPack for a notification service that needs to be really conservative with bandwidth and "alot of" messages running through a Raspberry PI board.
I haven`t run any performance tests so I can't, give you any numbers.
Most important for us was saving space.
And since every KB counts over thousands of devices we are able to save roughly 30 % on that costs.
However JSON is fast and good enough for 90 % of the use cases we have.
We use msgpax and its a wonderful library.
This was a bit of a rush job to get a prototype up and running fast I wish I had more knowledge on msgpack.
And I plan on doing some tests to get msgpack to speak to c++ just for fun.
I found this article some time ago and want to test it.