I'm going to say something I just thought of, something I just realized, and which if I'd said 6 months ago, I'd have called myself crazy.
The game speeds up the higher level you are.
We take for granted that as we learn a system, we get better at it and gameplay speeds up. True of all tabletop games.
D&D4 is different in that every class has its own unique powers and gets new ones all the time. So that regardless of how long you've been playing, if you try out a new class (and so far, there are no duds) your play slows down. You're essentially learning a new game.
It takes a while to learn what your character does, forget the system. The system is incredibly simple. It's the characters and their powers and what the designers intended that takes time to learn. As you gain levels, and learn the secrets of your dude, how all his powers work together, it gets faster and faster, even though your character is becoming more and more complex.
Eventually, for many characters, you won't even need the power cards anymore! Because when you begin, it seems like your new character has lots of options and lots of things he can do. You imagine that there's this vast array of ways to play. But you learn that really your dude does this one thing, with a limited set of permutations.
So, alas, I think the best thing for it is just play as much as you can. Don't let your players switch characters because they'll be learning a whole new game.