I regularly run a one-day invitational with up to twelve players, GMing it by myself; it's 1st edition AD&D, if that makes any difference.
Out of combat, it doesn't matter how long they take. In real life, it genuinely does take some time to get twelve people to agree on anything, even pizza toppings. If some players find the pace too slow, they have it within their power to speed things up within character ("I swear to Heimdall, if that cursed dwarf doesn't stop fixating on how we'll re-cut the Eye of Argon when we haven't even recovered it, I'll stick his axe right down his mineshaft, if you get my drift."). They're aware of a general background pressure of time ("if we wait until tomorrow, then although we'll have our spells back, the reinforcements our spy saw will have arrived"), but it's up to them to decide how to prioritise and trade-off.
If the players don't mind things going slowly when the game logic permits it, and they're having fun, who am I to care?
In combat, it's different. I tend to aggregate monster attacks, simplify their rolls, ignore details which will complicate and thus slow down the play; this is all done behind a screen, so I don't have to justify it to the players.
When it comes to the players' attacks, they're arranged in a horseshoe at the gaming table, and they're used to me sweeping my arm across the party in order (alternating the end at which I start), and as my arm reaches each player, I want a "declaration of intent" within 5-10 seconds. We roll party initiative en bloc on a d6; if we rolled individual segment-based initiatives on a d10, I'd simply run through the integers from 1-10 the same way.
If they don't get their declaration of intent out, they lose that round's action through dithering. If they have questions, perhaps "Could I shout a warning to the elf, drop the broken axe haft and attack with a dagger all in one round?", then a little horse-trading with me is OK: "yes, if the warning is five words or less, and you understand you won't attack until the very end of the round"; the cardinal sin is standing there open-mouthed when the arm points at you, with no clue what you want to do.
Yes, this can be hard on players whose attack is early on; but that's combat. If in the middle of 60 seconds' worth of cut, thrust, and parry, the opportunity for pressing the attack comes three and a half seconds in, and you're not ready for it, well, tough.
I also ruthlessly trim player interaction during combat. Anything you want to say to a fellow player had damned well better be done during your action; players who insist on having tactical discussions "in the background" know that there will be loss of action, -ve xp, and ultimately "the disfavour of the Gods" (ie, you're going to start getting very unlucky).
I'm sure it helps that I'm playing old-style D&D, where the flow of time is fairly simple; one thing that's put me off later editions big-time is the huge complexity of time flow, where players with the right spells/skills can insist on jumping into the time flow.
But on the whole, the players have been fine with this; they get the chance for long drawn-out planning and discussion sections, but when combat strikes, it's going to be fast, messy and disorganised. I've never been in melee combat myself, but my understanding is that it's often fast, messy and disorganised. At any rate, the palpable feeling of relief amongst my players when the last ogre surrenders and the players are still standing, and the subsequent back-slapping, congratulation and general signs of "decompression euphoria" suggests that this works for them.