We were using an online curriculum (k12.com that our free virtual school uses) that is designed for average students, so the content is quite dilute. There's just not that much "new material" in a few weeks worth of that curriculum.

You do have a point that when it does get to a lot of new material, as courses get higher, and using a tougher course like AoPS, then there won't be so much plowing through huge sections quickly.