Most 3e rounds, most of time, can go pretty quickly.
I think a lot of people are hoping 4e will be the "silver bullet" to solve all their problems.
In my experience, 3e combat rounds are usually quick and painless. The slow downs come from:
1. Player not knowing what to do. He's indecisive. Should I charge, or use my bow, or maybe I should throw a tanglefoot bag, or maybe I should drink a barkskin potion this round and run in to fight next round, or maybe... This kind of thing will still happen in 4e too.
2. Player trying to do unusual stuff. I will kick cinders from the campfire into the beholder's eyes, then try to stab it with the stick I've been roasting my marshmallows on. Do the cinders cause damage? Daze? Blind? Does the beholder take extra damage if the marshmallow on my stick is on fire? True, this kind of thing doesn't happen often, but when it does, it can slow down combat, and it will slow down 4e too.
3. Player tries to use a normal combat option that is hard to remeber/understand in game ters. Examples in 3e include grapple, overrun, and even turn undead. Some spells are hard to remember, either because they are complex (Dispel Magic) or just not used much. These things are difficult to remember exactly, so often it involves players digging up the manuals, finding the rule, then figuring out how to apply the rule. Sometimes arguments happen when players disagree. These can cause major slowdowns in 3e. My understanding is that much of this stuff has been drastically simplified, or even oversimplified, or just plain removed entirely from 4e, so these slowdowns shouldn't be too common in 4e.
4. Player just doesn't know the rules very well. How far can I move? What die to I roll to attack? Can I cast my spell then hit this orc with my sword? If I move this way, will those goblins get opportunity attacks? Etc. This happens in every RPG; 4e won't fix this problem.
5. Player controls too many creatures. Druids with multiple animal compaions, summoners who cast summon spells, etc. This means the player won't just move and attack, but instead he will move, attack, move, attack, move, attack, move attack, etc. This can result in a long turn for this player in 3e. It seems that 4e is limiting many of these options for players, so this may not be too much of a problem anymore in 4e.
6. Player isn't really paying much attention to everyone else's turns. He's watching TV, surfing the net, outside smoking, gone to the bathroom, chatting with another player about the latest South Park episode, etc. When it's this player's turn, he has nothing prepared, might even have to ask what's been happening. He wastes everyone's time while he figures out what to do because he's not prepared. This happens in every RPG; 4e won't fix this problem.
7. Distractions. Ofen the gaming table gets off on a sidetrack, chatting about any old thing under sun. Even the DM gets sidetracked sometimes. Even the player who should be taking a turn, moving, attacking, etc., might be sidetracked in this conversation. Sometimes the sidetrack goes on for quite a while, drawing out the entire time of the combat. This can happen in every RPG; 4e won't fix this problem.
8. Iterative attacks. High level 3e characters can sometimes make multiple attacks in a round, meaning several attack rolls and several damage rolls if the attacks hit. This is easy enough to handle without much slowdown, but few groups handle it, so the end result is roll attack, see if it hits, roll damage, then roll attack, see if it hits, roll damage, then roll attack, etc. This can slow down 3e combats and won't be a problem in 4e since getting multiple attacks in 4e will be very rare.
I'm not sure if that's the whole list.
You will note that some of those don't relate to the game system at all (4, 6 & 7). Some won't be fixed in 4e (1 & 2).
That leaves 3, 5 & 8 that 4e will likely improve.
So, in a nutshell, if you're playing in a group that has all these issues, then you can spend hours on a single combat in 3e, but in 4e, that same combat will probably take almost as long. But if you're playing in a group that rarely if ever has any of the issues, then your combat is already pretty quick and 4e won't make it much quicker. If your group is really quite good but issues 3, 5 & 8 are common, then 4e will probably make a huge difference for your group.
But, even with those, #3 can be offset by having the player become familiar with that combat option if he uses it a lot. If the player likes to grapple, have him study the grapple rules. Have him look them up during other players' turns so he knows what to do and what to roll when his turn comes up. This can eliminate most of the slowdowns caused by 3.
There isn't much to do about #5. When it happens in combat, it will result in that player having long turns. The only real way to minimize the impact is to not allow players to do this in the first place, otherwise you just have to suck it up.
As for #8, training players to roll all the dice at once really helps. Use color-coordinated dice. Red d20 and red d8 for the first attack and damage, blue d20 and blue d8 for the second, etc. When it's the player's turn, he rolls all the dice. Look at the red d20, add his attack modifier, tell the DM the modified roll. DM says "hit" or "miss", if it's a hit, look at the red d8 and add damage modifier and tell the DM. Then do the same for the blue dice. It still takes longer than when you only have one attack, but by doing this, 3 attacks don't take 3x as long as one.
