Good point lol.Rapid Heal prayer: 20 life points are recharged every minute as opposed to ten every minute.
in 24 seconds you regain 4lp without the prayer and 8lp with, therefore he would've had 750lp faster than this. He didn't have any prayers on and the video was edited.
If it wasn't fake he would probably also have an overhead prayer on as they're the most useful.

