[alfalfasprout]

For our use (serializing streaming market data from direct NASDAQ and BATS feeds) we found both Capn'Proto and Flatbuffers to perform similarly.

Ultimately we went with flatbuffers because we found the API much cleaner across languages (C++/Go), but it's ultimately going to depend on your use case which one you use. Performance is pretty much identical. We populate our own custom structs from the capn'proto/flatbuffers structs anyways so we're never really doing zero-copy. That said, since both of these formats transmit numerical data as little-endian memory-representations of ints/float/longs/doubles their performance is fantastic.