The problem isn’t that you can’t adjust the leveling speed, the problem is the WotC adventure paths try to cover too big a level range for the page count. “A level 1-10 adventure!” I did something of similar length from scratch, and it was level 3-8.
Is page count really what players care about, though?
I would put player experience before page count, and probably think of it in terms of cost - "player enjoyment per dollar" basically.