Not having been to XP, but playing a short game using the characters and some kobolds, I can say that it takes a bit to get used to when you're still reading each power to decide what to do. But I can really, REALLY see it speeding up once you know what your options are off the top of your head.
I expect a few sessions, or even a few encounters will have it cruising along at a nice pace.
It also seems that since everyone gets to act with less out-of-game time between actions, it feels faster. So, instead of waiting 15 minutes for your Tempest to make his 14 attacks per round, you've got much less time between his action and yours, thus you're spending less time sitting around doing nothing.
Same. One thing that still doesn't sit well with me when playing 3E, especially at higher levels, is that fights can often be decided in a couple of rounds. I've mitigated it through some house rules, which unfortunately does make all combat take a bit longer in real time.
Also remember, 3.5 encounters were fine at level 1-3 (or whatever), the problems come in at higher level. If 4e combat doesn't take exponentially longer with every level, I will be very happy.
Consider also that the preview format is reportedly a delve format where the DM is keeping a very tight pace. My experience is that most of what takes time in the current system is discussion digressions, and people deciding to take their actions by committee. The delve format doesn't have time for any of that as you have a set number of minutes and you're done whether or not you finished all of the encounters.
Thus, regardless of how fast or slow the game system runs, it will probably be faster than many players experiences in other contexts.
I think in general, there are more rounds in the same amount of time, but 1st-level characters have fewer powers than higher level characters. Once you hit your "cruising altitude" number of powers, it does take a bit longer to decide. (Still faster than 3.5, IME).