We use flatbuffers extensively in our message passing framework, and they are extremely fast if you take care with your implementation. They have a few features that make them especially useful for us

1) The flatbuffer parser can be configured at runtime from a schema file, so our message passing runtime does not to need to know about any schemas at build time. It reads the schema files at startup, and is henceforth capable of translating messages to and from JSON when required. It's also possible to determine that two schemas will be compatible at runtime.

2) Messages can be re-used. For our high-rate messages, we build a message and then modify it to send again, rather than building it from scratch each time.

3) Zero decode overhead - there is often no need to deserialise messages - so we can avoid copying the data therein.

The flatbuffer compiler is also extremely fast, which is nice at build time.