If you run a lot of terminal tabs or scripts that all need to make OpenSSH connections to the same server, you can speed them all up with multiplexing: making the first one act as the master and letting the others share its TCP connection to the server.
If you don't already have a config file in the .ssh directory in your home directory, create it with permissions 600: readable and writeable only by you.
Then, add these lines:
Host * ControlMaster auto ControlPath ~/.ssh/master-%[email protected]%h:%p
ControlMaster auto tells ssh to try to start a master if none is running, or to use an existing master otherwise. ControlPath is the location of a socket for the ssh processes to communicate among themselves. The %r, %h and %p are replaced with your user name, the host to which you're connecting and the port number—only ssh sessions from the same user to the same host on the same port can or should share a TCP connection, so each group of multiplexed ssh processes needs a separate socket.
To make sure it worked, start one ssh session and keep it running. Then, in another window, open another connection with the -v option:
~$ ssh -v example.com echo "hi"
And, instead of the long verbose messages of a normal ssh session, you'll see a few lines, ending
debug1: auto-mux: Trying existing master hi
If you have to connect to an old ssh implementation that doesn't support multiplexed connections, you can make a separate Host section:
Host antique.example.com ControlMaster no