Have you considered implementing training times? One of the basic conceits of the game is that, after a certain point (after you reach level 1), you need real-world experience in order to improve at all. And while it might make more sense that it takes three months to go from level 1 to level 2, it's impractical from a game-play standpoint to require the players to go through ninety adventuring days between each level.Personally, I don't think that adventures covering a few months of in-universe time should bring characters from level 1 to high levels. High stakes adventures covering a short period of time should involve high level characters, so that they may acquire skills/power/whatever as the story develops and be able to face the big threats at the end, without every single character being basically a genius/prodigy of his craft, capable of hyper fast learning.
Have you considered implementing training times? One of the basic conceits of the game is that, after a certain point (after you reach level 1), you need real-world experience in order to improve at all. And while it might make more sense that it takes three months to go from level 1 to level 2, it's impractical from a game-play standpoint to require the players to go through ninety adventuring days between each level.
As I mentioned earlier, an adventuring day is about the length of one dungeon, and a best-case scenario might let you get through a whole dungeon in only one session. If you play once per week, that would be almost two years of regular gameplay for each level, or 30+ years of real-time before you reach level 20. That's probably a bit slow, for most groups.
If you add in a month or three of training time before you can level, you can get a more believable ascension rate, but the cost is that you'll need to plan extended downtime into the game. It would simply be impractical to try and level up in the middle of a quest. And that's perfectly reasonable, if you want to do that, but it really does only work for certain types of games. (Personally, I don't see why anyone really needs to level up in the middle of an adventure. It solves a lot of problems if you could only gain levels between adventures.)
Yes, introducing training times is the first thing that comes to my mind, when trying to figure out how to moderate the hyper fast learning. The thing is, when the clock is ticking and the bad guys threaten to destroy the world, and some random level 1 mooks are supposed to be one of the main forces standing between their world and the uber evil of doom, and when they have to acquire the ability to face those uber evils in such a short time, well things start to get weird.
But, at that point, why are the PC's starting at 1st level? If there is an Uber Doom on a clock, and the PC's need to be level X by the time they face the Uber Doom (I am SO going to name my next bad guy that) why not just start them at that level?
What's the point of going through all the motions of adventuring to level up, when the point is to be level X by Day Y?
Uhm, that was my point: start high stakes adventures at higher levels. You should ask WotC why they are making ''save the world from something very dangerous stories'', setting up a situation where the Uber Doom can't be left unchecked, and where therefore the PCs don't have much time? They are making this kind of Adventures. All of them have the PC preparing to face/racing to prevent the summoning of some uber evil of doom, and all of them do the level 1-15 thing.