For the sake of completeness, here is a run-down of a typical British execution in the 20th century.
Minor details would vary from prison to prison, but all condemned cells and execution rooms were to the same pattern: a larger-than-average cell containing a standard prison bed, a table and three chairs (two guards were with the prisoner constantly in the time before the execution), the usual cell door into the corridor, and a double door in another wall, leading into the execution room. If the prisoner asked about the double doors, he would be told that the cell he was in had originally been a store room (which was often true – several prisons were built before the system of execution was formularised).
The execution room had a large double-doored trap in the centre of the floor, operated by a lever (similar to a railway signalman’s lever) to one side. Across the ceiling above the trap was a heavy wooden beam with a large hook in it, centred above the trap.
On the day before the execution, the executioner and his assistant (who were drawn from a small pool of officially trained part-time freelancers) would arrive at the prison. They would be given the prisoner’s height, weight and age, and would be allowed to discreetly observe the condemned man to gauge his build (which may have some bearing on the drop).
At some point that afternoon, the prisoner would be taken from his cell for exercise. During that time, the executioners would make their preparations: the trapdoor would be tested and the length of drop set up using a table of calculations first formulated by Victorian hangman James Berry which set the length of drop based on the person’s weight to ensure a clean break of the neck. The rope was a 3-4 feet length of hemp with metal eyelets woven into the ends – one end was passed through the eyelet at the other to form the noose. The part of the rope which went round the prisoner’s neck was sheathed in soft leather to ensure smooth running through the eyelet and to prevent tearing of the skin. The other end of the rope was attached to a length of chain which was hung from the hook in the beam – it was the chain which governed the length of drop. Finally, a sand bag of the same weight as the prisoner was attached to the rope, dropped through the trap, and left to hang for several hours to stretch the rope.
That evening, the executioners would return to the execution cell (quietly – the condemned man was back in his cell next door by now) and remove the sandbag, re-set the trap, and coil the rope loosely, tieing it with thin pack thread to the overhead beam so that the noose hung at head height.
On the morning of the execution the prisoner would be brought breakfast, given a glass of spirits (traditionally rum) and receive a visit from the prison chaplain. Shortly before 8 am, the executioners and witnesses to the execution would gather outside the condemned cell. As the clock struck eight, the witnesses would proceed into the execution cell, while the executioners would enter the condemned cell. The prisoner would be sitting at the table in the cell, with his back to the door from the corridor. He would be asked to stand, and the executioner would pinion his arms behind his back with a leather strap around the elbows. He would ask the prisoner to follow him, and lead the way through the double doors into the execution room which had been opened by the prison officers as he entered the cell. Within a few steps, the prisoner would be standing on the centre of the trap. The executioner would fit the noose around his neck, with the eyelet under the left angle of his jaw, and put the hood (a plain white cloth bag, lined with black material to make it opaque) over his head – his assistant meanwhile, was fitting another leather strap around the prisoners ankles. As soon as the hood was in place, the executioner would step to one side and operate the lever to drop the prisoner through the trap.
After a few minutes, a doctor who was one of the witnesses would go down the flight of stairs into the room below, test the hanged man’s heartbeat and pronounce him dead. The body was left to hang for another hour to ensure extinction of life.
While thirty seconds from leaving the cell to being pronounced dead sounds improbable (I doubt the doctor would be in that much of a rush, and it would probably take longer than thirty seconds to ensure the lack of a heartbeat using a stethoscope), the whole process was indeed very quick. Thirty seconds from the executioner entering the condemned cell to the prisoner dropping through the trap would probably be normal, but there were occasions when it took less than ten.