Two different games, two completely different experiences.
In my sister-in-law’s game, we fought Nas-something-or-other from Tomb of Annilition. Big fight, with a variety of yuan-ti and a couple Thayians. The fight dragged, taking over two hours because the DM wasn’t familiar with the stat blocks, hadn’t practiced the fight and had to look up several rules on the fly whilst taking agonizingly long to decide what each NPC was doing (and there were about 14 of them). We won, but I had lost interest somewhere about half-way through the fight. I think the whole thing lasted 3-5 rounds of combat or so.
Conversely, in our Theros game, our group was taking on a demon supported by waves of harpies. We fought our way through four floors of waves of harpies until we got to the top. We were a bit battered and concerned we didn’t have the firepower to take the thing on, but we had to try. We used everything, one of our own PCs got mind-controlled and turned against us (the raging barbarian, of all things!), but we managed to break the mind-control and a lucky crit from the freed barbarian took the demon out. At the end of the fight, we were out of spells, short rest abilities and you could have counted all four of our hit points on both hands. All five floors and the massive boss fight took less than two hours, and we were at the edge of our seat throughout it.