I'm not familiar with the adventure, but I would like to ask you: what does "too fast" mean in the context of a tabletop roleplaying game?
Does it mean too fast within the narrative? Don't worry too much about the players being ahead of the adventure's timeline. You may think things will be less tense if the players ace everything, but the swagger that comes from outperforming the par on a published module is more than enough emotional reward for most parties.
If you're concerned they're moving too quickly and missing out on something, certainly that's nothing to be worried about.
In RPGs, the players are the authors and audience of the adventure. The experience should serve their interests as gamers. As long as everyone's having a good time, there's little argument that anything "wrong" is going on at all. Ask yourself: what is lost in their approach to the adventure, and do they mind missing it?
I submit that most parties who breeze through without gathering additional context or information aren't interested in either, but are strictly concerned with the sense of achievement conveyed by fictional victories. Do you think this applies to your group--or do you think perhaps they want more out of the game and they just don't know how to get it?
In this latter case only, I'd recommend a simple conversation with your group outside of gametime. Ask if they're having fun, if there's anything else they're looking for. Perhaps suggest a few mysteries or questions they haven't explored and suggest something they might have done to learn more about it. If they really do care about getting a more complete experience, this is when it'll dawn on them they can contribute more as players. If they don't care, they'll probably keep doing what they're doing, and that's perfectly fine.
You did mention that they're moving faster than you're "prepared" for. If this is a logistical problem (and not just an expression of surprise), I recommend a more general approach during prep time. Anything that looks like something they won't investigate can probably be skimmed, and as long as you can put together a basic map, you're okay. Alternately, insert your own filler--combats with Bestiary monsters, minor sidequests, other published or amateur adventures that are thematically consistent, etc.