[ara4n]

The client-server API of Matrix only uses http+json as the lowest common denominator baseline for compatibility. Folks are more than welcome to implement custom more efficient transports like COAP/CBOR or whatever.

The server-server API is currently just targetting https+json just for expedience in getting started, but nothing to stop us negotiating more efficient transports in future - capn proto has come up a lot in conversation as a possible option. Handling signing is a bit more fun as we currently rely on signing canonical json, but surmountable.

As for binary transfers... well, random blobs are just handled as pure HTTP with a mimetype currently :)