Other people have given some good answers but I'll throw in a few thoughts on the matter myself.
The obvious answer would be to not slow down at all. If your players are blowing through your content and come back every week (two weeks, month, whatever) for more, that means you're doing a kick-ass job at keeping them engaged and playing. Slowing down might lead to them losing interest and you losing the momentum you've created.
Campaigns have a tendency to end in one of two ways. Either they end well, usually with a satisfying conclusion, or they just peter out untill they finally stop. You want to be in that first category.
Now, on to some actual practical advice because sometimes you need some tips and tricks for stalling out a game.
Good or at the very least interesting NPCs with some sort of goal or motivation are fantastic for slowing play down. Having a crazy old man, tavern wench with a chip on her shoulder, enigmatic elven wanderer, pompous mayor, and more show up and harass the PCs with their daily problems, side-quests or just some fun lore about the area is perfect for giving you some extra time to play around.
These kinds of speedbumps are a great way to give your players a bit of extra content in the form of a side-quest or an extra bit of lore for the lore-hounds among them. And if you do it right, you're creating NPCs that players will latch on to, come back to and build relationships with that will also give you more content.
Encourage Personal Goals
In addition to making characters more interesting, characters having personal goals give you tons of material to work with. I always encourage at least some level of personal quest building because it makes the players that much more involved in the world. As a bonus, it gives you a great means of judging what they want to see in a game.
In my current game, one of the PCs is trying to get herself crowned king of the elven empire. As a goal, it's pretty big in scope but it tells me the player wants to see more of the elves, more of their culture, more of their way of succession and so on.
Mysteries without answer
Putting some sort of either unsolvable or hideously obscure mystery in your game is guaranteed to take up a heap of time during play. However, you have to be careful not to make it too frustrating. Ideally, this mystery is enticing but not critical to the main story so it can't bog down or break your entire campaign. Perhaps half of all the elves left at some point. It's mysterious and weird but probably not critical. Maybe they left a load of clues as to where they went and the players can pick up on the story and learn what happened.
You don't need an answer to what this means or why it happened when you introduce it. Let the players learn about it and use their reactions to guide this particular mystery in a satisfying direction.
Think more in terms of audio-logs or easter-eggs than actual story points but important enough to make players seek it out.