This is partially fact, partially speculation:
Facts:
Pulling slowly/easily gives the atomizer more opportunity to vaporize more liquid for you, so the vapor is more dense, so you get a better hit and more flavor, because the same volume of air has more flavoring and nicotine (assuming you use nicotine) in it.
Pulling fast means less time for the vaporizer to vaporize, so you pull in more air, less vaporized
eliquid, and end up with a thin vapor that has less hit and flavor.
Speculation:
If you pull for a long time, the atomizer can get too hot and either give the flavorings a burned taste, or it can get dry for a moment and give off this nasty dry atomizer taste.
If you pull hard over and over again with little pause between, the atomizer wicking won't be able to keep up and you will get that bad dry atomizer taste again.