Hmm - I'll go dig out my encounter records and find out how many rounds each combat lasted. (That'll have to wait a few hours, though, as they're at home).
The shortest (and most surprising combat, from my point of view) was the fight against the manticore.
Round 1: The party enter the tower. The manticore wins initiative, and flies up, through a hole in the roof, and perches on the wall, and sends a volley of spikes at the fighter, hitting 3 times. (ow! ow! OW!) The other PCs move into position, and the wizard casts
Tasha's hideous laughter on the manticore. The manticore fails its saving throw, and lies prone on the wall laughing.
Round 2: The fighter climbs up the wall. It's a DC 10, and he has ranks in Climb. I love it when a player's skill choices are vindicated. The rogue tries shooting the manticore, but it has cover and is prone. (Not good). The wizard cleans off his spell component bag and looks smug.
Round 3-5: Whack! Whack! Whack. The fighter kills the manticore.
That was the shortest encounter of the day.
Cheers!