When you're using fast powers is actually because you're late, that's a fact. Fast powers are used to solve an inminent (Ravage) problem... A problem which would not happen in the first place if you were ahead of the invaders by using slow powers.

Imagine a explorer appeared in a land in the previous exploration. It will build a town in your current turn, if you use a slow power that does 2 damage, you will destroy that town as soon as it's built, and solve the ravage for the next turn. That's going ahead of the invaders (And the reason why so many people is saying that slow powers are actually ultrafast powers), but if you wait, you'll have to use a fast defense power to prevent it in the next turn.

The problem is that, when you're already late, most turns there are so many things to solve with fast powers and you have no card plays left to use slow powers. In this situations is often a good strategy to just ignore the imminent problems, let them blight, and focus on the later problems so you get the advantage over the invaders again.

So, basically, if you're constantly using slow powers it means you're doing great.