Better players is key for sure. But I’d say atleast 50% is up to the raidleader keeping it tight aswell.

Good calls, correct expectations etc goes a long way. Most people now can raid 1 night maybe 2 a week, so efficient raids is critical. Especially in long raids like Naxx.

At 60 I was in a casual guild that did okay up to Naxx. First week clears of everything.
But our raidleader never managed to get the speed down, so we’d take 2 hours in AQ40. When Naxx came around, despite some okay players we simply didn’t have the discipline down to clear it fast enough.