I'm currently using the JSON Rest API to get lots of transaction data from bitcoin. Unfortunately thats quite slow. I know there is also a binary format (see Unauthenticated REST Interface), but I couldn't find any description of the bin format. Is there any description of that format available?
For transactions? https://en.bitcoin.it/wiki/Protocol_documentation#tx
This is the internal format, for communication between Bitcoin nodes. As such this won't be as useful as the JSON format, since you will need to parse it on your own.
This is also equivalent to calling the RPC
decoderawtransaction with tx received in the hex format.