A deploy script was spending most of its time doing nothing. Each of the forty commands opened its own connection, and every handshake cost about half a second.
Three lines in ~/.ssh/config fixed it:
Host *
ControlMaster auto
ControlPath ~/.ssh/cm-%r@%h:%p
ControlPersist 10mThe first connection opens a socket; the rest reuse it. The script went from fifty seconds to under nine.
Two things to watch. The socket path has a length limit on some systems, so keep the directory short. And ControlPersist means a connection lingers after your shell exits, which surprises you the first time a firewall rule changes and nothing seems to take effect.
To close them by hand: ssh -O exit host.