My raid full cleared Naxx25 pretty quickly and we're far from hardcore. It took us two reset periods (maybe three), I think, but we only raid twice a week for three hours; if we had run more hours a week and weren't so limited on time, we would have cleared it in our first reset. Most of us were in blues and craftables. A couple people had ten man gear already, but not very many. I know I started still wearing my [non-sunwell] Tier 6 because I had been leveling another toon and had no time to find enough good gear to justify losing my set bonus.

In my experience, (a) a lot of people think they are not ready for Naxx until they are in full heroic gear/rep epics and thus start much later than "necessary" (assuming they want to raid and (b) a lot more people are finished with Naxx then think they are. The few raids I still know who are running it (assuming not an alt run) absolutely have the gear level for moving into Ulduar, but think they need to stay until every last person has every last blue replaced... which isn't the case. So I think those influence people's progress speeds quite a bit.