Even full grown and well trained adults will start tiring after about 2 minutes, especially if the patient is a larger person. Ideally it's done in 2 minute cycles, then a quick pause for heart rhythm check and shock if necessary, and a different person takes over compressions after the shock. These were mostly teenagers, so even more impressive. AEDs are really simple and will give verbal instructions once they're on, but still takes some knowledge to set up right. All CPR classes should include practicing with AEDs. Of course when that adrenaline starts rushing it can be hard to think clearly, which is why practice is so important.