Thanks Bdale and Chuck for providing some of the answers!
How do I reduce the heartbeat timing below three seconds?
The CDNC hartbeat control is in two pieces: (1) the pulldown for changing frequency of the heartbeat and (2) the button to enable or disable the heartbeat.
So: select the rate using the pulldown, then press the [Heartbeat] button to enable it. If you want to change rate then press the button to stop it, adjust the pulldown to change rate, then press the button to enable it once more.