I recall watching Mike Mearls GM a 1.5 hour live event with 8 actor/voice actor players that seemed a lot like a 5-room dungeon which played pretty fast... Here it is:
Thanks Quickleaf! Here are my notes from the video.
There were 8 players with 6th-level characters and one manticore.
The sequence of events was:
-They started with a few minutes of introductions, followed by what amounted to a summary of a (non-existent) session that got them to their starting point on a ship under a floating tower with flying potions.
-They entered the tower and had some social interaction, until a combat broke out with four redcaps.
-After the combat there was some exploration that involved looking into two adjacent rooms, and sending a familiar down a passage to another room (they abandoned that line of pursuit shortly).
-A second fight broke out with what appeared to be a normal beholder with a spectral appearance. This one took place both in the initial room and the vertically connected room that the beholder started in. Flying potions were still in effect.
-The video basically ended with the conclusion of the second fight. (The game continued.)
Things I noticed that can affect speed (either speeding up or slowing down):
-As mentioned, there were 8 characters and a pet
-The players knew their characters and the rules pretty well
-The DM, being the lead designer of 5e, knew the rules pretty well
-After they got going there was negligible out of character chatter
-In-character chatter started light, but picked up after the first combat was over
-The players often spoke in their character's voice, described mannerisms, etc
-At the beginning, each player acted completely independently with no planning or coordination, in-character or out.
-After the first combat was over they did a light to moderate amount of planning and coordination
-The first combat took about half an hour, the second a little longer
-Initially, the players had few questions for the DM except about range to target.
-After the first combat finished, and particularly during the second combat, more questions popped up, but I wouldn't say it was excessive
-During the first combat, the DM used an expedited side initiative (going around the table clockwise)
-The second combat really began a bit before initiative was rolled. One character and the beholder exchanged some attacks that would have taken more than a single round.
-The DM used set damage rather than rolled damage for monsters
-The DM fudged a 28 hp fireball to kill a 45 hp monster at the end of the first fight
-The DM was on the generously favorable to the players side in rulings on non-standard action attempts (such as allowing a
blindness/deafness spell to shut down 2 of a beholder's eyes when it failed it's save)
I consider my group pretty slow, and with 5 PCs, we likely would have taken a bit longer.
We would have taken a lot longer if you include that we would have actually played through getting the mission and traveling to the destination in a previous session(s), and that we likely would have explored downstairs before going into the beholder room, but let's just look at this session's events.
The initial time-frame before the first combat would have taken maybe 5 minutes longer.
The initial combat might have taken 6 or 7 minutes longer (I'm assuming we were properly matched, rather than overmatched because of lower number of PCs).
The exploration after combat, assuming we decided to go the way they did, would have taken up to about 8 minutes longer, unless we set off the beholder (probably pretty easy to do).
The second combat (again, assuming the beholder was weaker to match our party's relative power) would have only taken another 10 minutes longer than theirs I expect.
Their 90 minutes would have taken us around 115 - 120 minutes according to my estimates, so up to about a third longer.
My assessment is that this wasn't a particularly fast group.
You know what would really help us see the difference, is watching videos of different groups all playing the same adventure. (I'd love to see how anyone gets through the Lost Mine of Phandelver in 20 hours.)