From fall of 2007 to early 2010, I ran a 3.5E campaign set in The Kingdoms of Kalamar. As an homage to my terrific old DM from the late 90s and early 00s, I had an early climactic combat with the players defending the grove of a dryad against slavers and allies of the slavers, as he had a similar scenario in 2E days from a fantastic Kalamar campaign that started in the same location in game.
The good guys in my 3.5E campaign were 7 PCs that were level FOUR, plus half a dozen allied humans that were level 3 or 4, as well as the dryad (a level 7 cleric) and her unicorn mount, and a wizard the party had rescued, also level 7. The bad guys were four groups of orcs, about a dozen orcs in each group (so about 50 total); some half dozen goblin wolf-riders with them; plus, a group of human warriors protecting the evil priest leading the slavers (a dozen warriors, plus the level 7 priest and two lower level priests); a group of non-combatant slaves to carry the evil priest on his palanquin; some drow warriors, led by a drow duskblade; and, a nasty hill giant.
That 3.5E combat took three entire sessions to play out, and ended up with two PCs dead, one almost dead, and a major NPC squashed by a mega-crit from the hill giant.
In late 1998, we had a similar scenario in the same grove in Kalamar. However, this time, there were 10 PCs at level four instead of seven. And, maybe a dozen allied warriors instead of six. There was the dryad on her unicorn, but she was not a cleric, just a straight 2E dryad. The party also had a pseudodragon, three groups of pixies and two brownies on their side. I think the dryad had called a treant to fight for the party as well.
The bad guys were also greater in number, including about a dozen goblin wolf-riders, 80-100 orcs & goblins; with a dozen drow and several slavers in the lead. Oh, and there were three hill giants instead of one.
The 2E encounter ran the entire length of one session - a very focused five plus hours. I think we had one PC dead, two that were near-death and most of the allied humans were dead.
While my 3.5E group was not as focused as the old 2E group, it was a shock to me that while the combat scale was smaller, the combat took three times longer, even at lower levels. (the combat times even went up as the group went up in level in 3.5E as well)
Also, in case you were curious, the homage ended after that combat, as my group went in a completely different direction than the old 2E group.
I also think part of the reason was that by 1998/99, 1E and 2E had been out for a good 20 years, so everybody in the group knew the system backwards & forwards. 3.5E was only a few years old in '07/'08, so nobody had a 20 year history of growing up with the rules.
So far, I've found 4E combats to be speedier than 3.5E, but my group is only up to level five now.