One consideration: When the neck snaps correctly the spinal cord is also severed, resulting in the deadening of any sensations or the inability for the transmission of sensations either up or down the spinal cord. Thus, I'd think there would be no pain if the hanging is done correctly, and the organs such as the heart would fail fairly quickly as the brain can no longer control anything. Just a thought.
I believe the heart is an involuntary muscle. I read that after a hanging, the heart may continue beating for up to 20 minutes. But, if done properly, loss of consciousness (if not death) is instant, because it snaps part of the brain stem.