The problem with con games is that, regardless of whether they are an RPGA game, a independent game, or something else entirely, you need to focus on what your play experience is going to be.
When you only have 4 or 5 hours to introduce the characters, the plot, play the game and come to some sort of conclusion, something is going to have to be skipped or not focused on.
In an average D&D game, you want to give players chance to use some of their cool powers. My couple of discussions with WOTC employees about adventure building and design have always said the same things: Players, as has shown by their market research, want to use their cool powers. They see a character sheet that lets them teleport an enemy 4 squares and hit them for 2d10 damage, they want to try it out. Part of the fun of having those powers is getting to show off. You have powers that other classes don't.
So, one of the primary mandates in RPGA games is that there has to be at least 2 combats in the adventure per 5 hour long slot. Most adventures use 3 combats. Each one generally takes 1 hour to 1 and a half hours. Generally, that means 3 hours used up with combat. Leaving about an hour or 2 for role playing, plot, setup, take down, and so on.
The reason 3 combats(and sometimes 4) are normally used is that (at least in 4e), it takes about 3-4 combats to deplete someone's healing surges to close to 0. It creates a growing sense of trepidation and tension as you get closer and closer to dying, creating urgency. That's the same reason it's heavily recommended that you not put any point that a long rest can take place during an adventure. Generally that means setting up a plot reason why the entire adventure needs to be completed in a day.
Roleplaying takes a lot of time. Or at least it can if you let it get away from you. Which is why not a lot of con adventures encourage it. A simple conversation where the characters introduce themselves that is full of roleplaying can go on 2 hours by itself. When that is the entire time you have for all of the non-combat portions of the adventure, you don't want to encourage that.
I will generally cut people off and encourage everyone to concentrate on the plot. I try to summarize things in as few words as possible in order to move the game along more quickly. If people pay close attention during the combats and take their turns quickly(something I can't really guarantee, since it depends on the players), then there is extra time for role playing and I'll encourage it more. Pacing is one of the key skills of convention DMing.
On the other hand, rarely will I cut a battle short for time unless it is extremely obvious that the PCs have it wrapped up without any real chance of taking significant damage. This is because I've seen players get really annoyed at feeling like battles ended anticlimactically. I know I've felt it as a player before. I was just thinking "Cool, I'll use my daily on that one there, which lets me do extra damage to everyone within 2 squares of it for the rest of the encounter. That's going to be awesome." when the DM says, "Yeah, you have this battle, the enemies surrender." It...isn't very much fun. I enjoy killing monsters.
As for why people go to conventions. Everyone has their own reasons. I want to get XP so I can go up levels and get new powers. I want to be told an interesting story, be given a chance to be the hero, and use my powers to kick monster butt. I like knowing that I was "that guy who saved the town from the attacking monsters". To me, the time I spend saving the town from the monsters IS roleplaying.
I don't know where your patience level is. I'm used to dealing with people who don't decide what to do until it's their turn. This actually ties into the thread about how long your turns take in an average game. I'm used to a 3 round 3.5 combat taking about an hour and a 6 round 4e game taking about an hour. Both of these are with 6 players. About 50% of the time is taken up by the DM. Which means that each player's turn takes about a minute in 4e and about 2 minutes in 3e. I'm not sure if that's too much for you. But it's perfectly normal for me. Some people take less time, others take more, but it averages out.
I've just learned that it's not worth it to complain if someone is taking a while. You complain, they get angry because they think they are moving fast enough, then it ruins the mood at the table and no one is having fun anymore. I have a friend who will complain about the speed we are playing games nearly every session. It's become no fun to play with him.
As for the rest of problems with people. Well, people come is a variety of shapes, colors, sentiments, attitudes, patiences, and personalities. You kind of need to deal with that when you go to a convention, because all of the different types will be there.