> Synchronous is almost always more efficient.
Synchronous is almost always more efficient when there is very close to zero processing time necessary between receipt of communication and response to it.
As you get away from zero processing time, the efficiency of synchronous communication drops very, very quickly.
Also, that's assuming 1-on-1 communication. Even when a synchronous exchange is between two parties is efficient, every unnecessary party whose work is blocked because they are a party to the meeting drops the overall efficiency considerably.
Which is why meetings tend, in practice, to be very inefficient if they aren't well planned: while synchronous communication can be efficient for certain communications and a properly chosen set of participants, meetings often involve lots of people unnecessarily bound up waiting for other people to complete synchronous exchanges, and often involve subject matter where asynchronous exchanges would have been more efficient even for 1-on-1 communication.