In Guild Wars 2, you are able to revive other downed players and pets/NPC allies. To revive someone, you channel the revive skill and as you channel the skill, the person or animal you are channeling revive on gets ticks of health back until the revive bar reaches full. When it reaches full the person or animal is revived with half health.
I noticed that on different occasions the health ticks to revive someone is different. Sometimes it takes only a couple seconds to revive while other times it takes up to 20 seconds or so. What affects how fast you revive someone?