I feel like a party should be able to end an encounter against an equal CR creature in three rounds or less a good deal of the time. Just by math and damage per round, this makes sense.
What doesn't make sense is when a GM just has these kinds of encounters, with time between to heal, etc...
The specific encounter may only go three rounds, but you could be in combat much, much longer as things keep coming or you have to navigate the conditions.
Yep, navigation. What of encounter distances and the melee/ranged capabilities of the party?
So a melee party will have a combat dragged out and be skirmished up, if the opponents are far away and have good movement/cover/difficult terrain on their side.
A blaster/ranged party can have a harder time if they are rushed/ambushed/grappled/ranged weapons sundered, so going close will make them less effective and the fight goes longer as they try to get control, or get themselves killed.
Incorporeal ghosts and invisible opponents can lead to a fight dragging out.