My experience was that combat initially took a while, but it sped up a lot as our group got used to it and learned smart tactics. 4E is much more of a "team" game than previous editions; it's all about working with the other PCs to set up advantageous positions. The fighter needs a chance to get close to a bunch of enemies and lock them down; the rogue needs room to flank; the wizard needs enemies bunched up and kept at a distance. If you don't keep an eye on what your allies are doing, combat will take a lot longer and be a lot more dangerous.
Admittedly, my gaming group is chronically short of players, which may be part of it. We usually have 3-4 PCs instead of the standard five. On the other hand, I tend to throw pretty tough opposition at my party, and so does the other DM in the group. I think our speed record for finishing a serious fight involved a 1st-level wizard, warlord, and rogue, versus an orc Eye of Gruumsh, an orc raider, and an orc berserker.
The PCs got highly advantageous terrain, time to position themselves, and the element of surprise, by way of making up for the encounter being almost 4 levels above them. The wizard was on a high ledge, the rogue hid behind a boulder, and the warlord was standing just behind a choke point, waiting for the orcs to close.
First round, the wizard cast sleep as the orcs approached. She was able to catch all three orcs in the area, and hit the raider and the berserker. The raider threw an axe and missed, and then both the raider and the berserker failed their saves and dropped.
The warlord moved out to engage the Eye of Gruumsh in melee, while the rogue popped up from behind the rock and began slitting throats. Neither the raider nor the berserker managed to snap out of it in time, and the rogue quickly dispatched them both. After that, beating down the Eye of Gruumsh was a fairly trivial exercise; normal controllers do not do very well with no backup against three PCs. I think the whole thing took maybe five or six rounds.