There is no competetive advantage in playing fast. Sure, you might enjoy yourself more, but it has to be obvious to anyone that playing more slowly and thinking through several scenarios can't be worse than going with your first intuition (of course there are cases where you talk yourself out of the optimal play, but that's not the fault of thinking, but of thinking wrongly).
The problem is that there are very few people who have the tools to think about these things productively and the marginal value is often pretty small compared to the intuition of a player with similar skill. That said, I think that methodical approach has the biggest chance rise the skill ceiling of the dominion community, especially if the trend continues that we get more and more control of our decks.
edit: Personally, I also play pretty fast most of the time, but I think I'm wrong about it
Also, the correct speed to watch dominion videos is 1.25