Think about the Teleport shout, I can't wait to fight a dragon that's teleporting around! hehe.
Considering that shout is intended for stealth, I wouldn't expect dragons to use it much, a giant, flying, fire breathing lizard is probably not the most stealthy creature in existence.
As far as dragons using the slow time shout, I don't know if it will happen or not, it sounds like dragons use the same shouts as the player, but does that mean they can use all shouts the player can learn? We'll see. As to how it would work if dragons use it, I'd assume it's like others have said, with the dragons moving super fast, as it would seem to an observer if you were able to slow time while moving at normal speed, unless it's actually like your typical bullet time in that it slows time for everyone, including you, and just serves the purpose of giving you more time to react to things, in which case, one wouldn't be able to have dragons in a way that really makes it obvious to the player that it's having an effect, but I kind of like the idea of moving at normal speed yourself while everyone else is slowed myself, and if it's like that, it would certainly be challenging if dragons used it, but it wouldn't make them invicible, after all, it's not like the shout would last forever, and I'd assume dragons also have a cooldown timer for their shouts so they couldn't keep spamming it, not to mention if you know the shout as well, I'd assume that you could also use it to move at the same rate as them while the surrounding world is slowed.