I've just finished playing through KOTS with 6 PCs (and 6 players)- we played out 25 encounters in total, we played 1 to 3 sessions/week, the shortest being an hour long, the longest being approx. 3 hours. We did lots of cool role-play- we were finished in ten weeks, 26 actual sessions of play, although again at an average of ninety minutes/session.
And I added monsters to make up the numbers because they had six PCs, or at one point seven PCs, I killed a few of them en route however.
Of the six players one of them had played D&D in a previous iteration, the other five had never ever played a rpg (save on a computer) in all of their days- they didn't know anything.
I think what I'm saying is it can be done...
By-and-large I tried to make each encounter spectacular for one reason or another, have the bad guys go for the players throat (where possible and doable) in round one. Or else impose time conditions- any more than six turns and the next encounter will be alerted, or else rewarded PCs with extra APs for doing X, or else do atmosphere in bags with visual/audio effects.
Players get a +1 To Hit bonus for RP in fights- speechifying, or else talking up their action in an amusing/interesting way... if they're quick then they do extra damage. I started out saying that each PC must get through their turn in 3 minutes, were down to ninety seconds- and the players are much more complex with more choices (they're Level 4 and in Thunderspire now).
I think if you want quick (more interesting) fights in 4e then you have to plan for it, and make some rules... And you have to have players that are up for it, and are going to bite the bullet and not think through every option. 4e is a tactical game and players get bogged down in 'if I do this... he can...', don't (or at least don't worry too much until you get your head around all the options)- pretend your heroes slightly out of your depth (a bit of mild panic is good), then go ape crazy with sword and spell- overkill works.
And as a DM narrate a good game- heads should explode, tentacled beasts should appear to drag the bad guy off, one of the PCs should start talking in voices. Kobolds that hit something should dance for a turn, or else squabble with each other. Goblins should be good at sneaking and better at swearing (and from a distance). Every leader (Kobold, Goblin, Hobgoblin) on the way should be a mixture of Napoleon, the Godfather, Colonel Kurtz, and Wile E. Coyote.
And there should be cut scenes, and flashbacks, and it should be a movie, and ninety minutes of interesting with a cliff-hanger is much better than five hours of... and you know this of course. I'll get me coat.
Cheers PDR