In my experience 4E combat is about 25-50% faster than 3.5E. It is true that there are generally more rounds of combat involved, but I find that the turns go much more quickly due to reduced complexity of the rules.
I've run every single encounter in KotS, and some additional baddies from the MM for a level 3 and level 4 party. Dragonborn Pally, Elf Ranger (20 Dex), Elf Cleric (20 Wis), Eladrin Wizard (20 int). The innate +5 to hit seemed to often make the difference between a hit or a miss, and the extra damage adds up of course. The party is very experienced and tends to go for a no-holds-barred maximum-damage strategy, focus firing and flanking, coup-de-gras on the sleeping enemies, etc. The longest fights were generally due to a combination of bad rolls and foes wielding shields, the 18-20AC range is tough to hit at low levels, and heavy shields are fairly plentiful.
As a DM, my style is to do everything possible to keep things moving. I try to limit each players' turn to under 60 seconds, so 1 full trip around the table (1 combat round) takes a maximum of 6-7 minutes. I keep the initiative order visible to the players and instruct them to take their turns one after another, without waiting for me to call on them. Everyone uses printed cards for powers and magic items so it is easy and expedient to keep track of what has been used, and the PH tends to sit unused on the floor throughout the fight. A balanced combat takes anywhere from 25-40 minutes. Big fat solo mobs take longer of course, and defensive, terrain-heavy fights can also drag. On the other hand the wizard does a good job of nuking minions so the mass-minion encounters are usually over in under 30 minutes.
Bottom line: 4E is fast and furious. Experienced players with a commitment to moving through turns quickly should finish more rounds in less total time than 3.5E fights.