I've also seen entire sessions where all the characters do is fight each other.The biggest variation, by far, in session time versus adventure "progress" is the amount of emphasis the players put on theatricality and thespianism. That's not a judgment or anything, my groups do plenty of it, but it absolutely makes the overall momentum of the adventure slow down tremendously, like up to an order of magnitude slower.
Lots of in-character banter, strategizing and making decisions in-character, scenes that are entirely about characterization and not about plot, are also time sinks. Beloved time sinks, usually, don't get me wrong (I'm really not denigrating playing in character, I swear! I do it all the time), but they mean the game will go slower.
Doesn't bother me - there's always next week for the survivors to recruit replacements and get to the adventure.
To the OP: the biggest factor IME other than in-character roleplay - and someone else already hit this upthread - is granularity of detail. As both DM and player, I'll often err on the side of too much detail rather than too little; and if doing so makes things take longer I don't care. I'd rather go into depth than skip over stuff, especially if there's any chance that the skipped-over stuff might have caused - or had the opportunity to cause - the PCs to do something differently.
Another factor - and one that the DM can directly control - can be the DM's own verbosity. Go into detail, sure, but when two words will do don't use ten. This is something I keep having to remind myself.
And, IME adventures generally tend to take longer on average as character levels advance; as both DM and players have more to think about in terms of character/monster powers, abilities, and moves.
Long term, our typical session count per adventure - including post-adventure downtime and treasury division, which is often a session all to itself - is about 8-10; with some as short as 3 and the longest getting well up over 40.