The cause of death from crucifixion was usually asphyxiation - the inability to breathe. Normally, breathing is a reflex action. One doesn't have to pay attention to the fact that the diaphragm moves up and down to cause the intake of oxygen. But when hanging on a cross by the arms, a person is in the "exhale" position and has to pull himself up to be able to take in air.
Sooner or later, with no food or water, the person becomes exhausted and, unable to breathe, suffocates.
This form of death is long and torturous, and could last three or four days. Executioners sometimes prolonged the agony by placing a wooden support at the hips and/or feet, enabling the victim to push upward ... until they were too exhausted even to try.
If the scourging beforehand was especially brutal, the victim would have less strength, and death would be hastened. Perhaps, because Jerusalem was crowded with Passover pilgrims (a time when disturbances often broke out), Pilate ordered a more severe scourging, and Jesus died more quickly than usual.
- Little Black Book, Diocese of Saginaw