I often work on many machines and end up with multiple sessions to each. Therefore I finally got around to checking out the ControlMaster and ControlPath configuration of ssh. I added the following to my ~/.ssh/config which is for a virtual machine I use on my laptop, so no network issues in play.
Host vm
Hostname 192.168.100.101
User dan
ControlMaster auto
ControlPath /Users/dan/.ssh/master-vm
Then in a terminal I ran the following, twice. Once on its own then with an ssh session running in another terminal.
for X in `seq 1 5`; do time ssh vm "pwd; uptime; hostname;" >/dev/null; done;
Without avg.: 0.268s
With avg.: 0.035s
I think that is quite impressive.
For the raw terminal output, please see here.
Update
Of course you can do this dynamically.
Host * ControlMaster auto
ControlPath ~/.ssh/master-%r@%h:%p
Then each socket will be used only when the user, host and port are the same.