[btilly]

Not always, granted, but the format really can matter.

Grabbing a random benchmark, look at http://www.maxondev.com/serialization-performance-comparison.... It is in C#, but every language is going to give similar results. An efficient on the wire format like protocol buffers is a bit faster and a bit smaller than a sane text format like JSON. But XML is several times slower and several times bigger. He didn't test pretty-printing the XML, but that makes it several times slower and bigger still than plain XML.

This adds up sooner than you'd think. For a start there is a noticeable timing difference between an RPC call that fits in one packet and one that has to be split across two.

If you've got an application that spends a sizable portion of its time in serialization/deserialization, it is obvious that the format chosen will affect maximum throughput. (Many distributed applications spend most of their CPU time doing those two things.) It is harder for the format to have a noticeable impact on latency, but it both can and I've seen it happen.