In 3.x, we leveled up pretty fast at low levels, and it gets slower at mid levels, then speeds back up at high levels (as the DM gets tired of the grind and starts handing out levels every few sessions).
In 4.0, I think leveling up is actually slower at lower levels. In 3.x low level fights were very fast, you hit stuff, they die, encounter over. In 4.0 fights go longer at low levels. I'm thinking I'll need to speed up the leveling up because the required number of encounters between levels is a bit too much for our play style. There are natural adventure resolution points in a campaign, and I like to have people go up a level based more on accomplishments than XP, so I'm sticking with my old ways when it comes to determining when it's time to level up, which never really was monster XP based.