While people complain about this, I played a lot of D&D 3.5, and somehow with all the sourcebooks and tables for everything, combat was snappy. DMs looked up the relevant rules for each fight ahead of time, everyone knew at least the basic the combat mechanics (except grappling), and players kept track of the relevant rules for their characters.
I assume this is a combination of the critical role people doing their glacially-paced combat and average D&D players not really knowing the rules (and thinking they had to stick to them). Also, I think this relies on players not caring that much about doing the "optimal" move in combat and instead role-playing through it.
3.5 is CAPABLE of going as fast as B/X but it never does for the general reasons
@Judge Dredd laid out. But if you have a party of martials, maybe a
wizard sorcerer and a cleric loaded up with just heals and everyone did their prep and memorized their numbers, it can go fast.
But so can 4e. Or 5e.
(those just have more numbers to memorize)
As a DM, I would regularly secretly adjust the HP of enemies to make fights feel the way I wanted, too - if it's a boss, someone's getting at least close to death before the PCs win. Sometime around the middle of the 3e-3.5 era, D&D players forgot that the rules were a canvas for the story.
I will only do this in very, very cases where I have done made a fuck up or miscalculation.
If you just change shit behind the scenes there's no point in tracking HP and you might as well play a narrative game.
Yes, everyone here is have fun but they all sat down to play with a rule set.
That said, if its a long combat at the end of the night and an enemy has like 2 HP, I might decide there's been a rounding error. Or a player with almost no HP and no more heals in a do-or-die just rolled a crit, I may decide they did such a notable feat their diety altered the flow of time and removed some HP from the boss.
The GM and I were talking about it not too long ago. 5e is simpler than 3.5e, so it can't really be an issue with rules complexity. At most, 5e players have more resource-limited abilities they can use (stuff that recharges on a short/long rest), but choosing when to Smite or Action Surge shouldn't take more than a couple of seconds unless you're playing with people suffering from crippling analysis paralysis.
In the end, we reached the conclusion that it's not the games, it's what around them that's changed. Specifically, smartphones, and later on VTTs. By the time 5e was released there were already way more distractions around a table than at the end of 3.5e's product cycle. And after the coof in most cases there's not even a tablet anymore, but a browser/chat program/game you're going to be playing with on the side while nobody in the phone call is the wiser.
Pair that with the fact that the people who play 3.5e or even AD&D or BECMI these days tend to be the kind to enforce paying attention around the table a lot more harshly, and 5e feels glacial by comparison. On the other hand, I've played plenty of 5e with people who were playing attention and the turns go by very snappily (more so when playing theater of the mind since there's less counting grid squares). Combat encounters still drag on like a motherfucker, but that's because everything has just so. much. HP.
Its more than just attention spans. 5e is the Common Core of TTRPGs. When you have a party min/maxing there are ton of temporary and situational bonuses/penalties to keep track of. It is no where near as bad a 4e party who is specced for maximum GM annoyance, but its still very bad. And due to this, the exact values of number matter.
3.5's bonuses were less dynamic - you were at +4 damage this fight or you weren't. Flanking was about the only thing to keep track of.
Again though, you're just really picking your poison. OSR stuff does a lot of abstract language "Effects the entire room" - but doesn't define the maximum size of a room so you have your powergamer standing on the roof of the inn trying to argue his aural effects now have unlimited range because the outdoors is one big room.
The more time I spend looking and running B/X and looking at AD&D/2e, the more I understand WHY 3e/4e/5e are the way they are as they tried to solved problems the earlier version created and making new ones.
It just comes down to which set of problems you prefer.
Edit: and B/X having 50 years of playing testing on common pitfalls and what corners need sanding the most.
