AbdulAlhazred
Legend
In practice, I haven't necessarily found this to be an issue.
It depends a bit on the at-wills: in my game the Sorcerer's default attack is an at-will Blazing Starfall that is a burst 2 which has +40-something adds, and so is (damage wise) not very different from the same character's encounter powers; and the paladin is built around Enfeebling Strike as the default attack.
In my experience, it is the terrain element of framing that is much more significant than whether the PCs use at-wills, encounter powers, terrain/improve attacks, etc. That doesn't offer any sort of general guideline, of course - but it's one bit of anecdotal information.
Yeah, my feeling is wave encounters can often go faster. If the party has one or two meaningful targets at a time to focus on, they tend to die FAST! Its unlikely that when your bow ranger AND your rogue start slagging a standard that it is even getting in a shot unless it got a good initiative roll. The cavalier, the wizard, and the fighter aren't likely to leave a 2nd one standing either.
Obviously, if you're cranking up the monster level to make up for the less demanding encounter format, then each wave may well take 2 rounds (and many will anyway), but its not guaranteed. Certainly the tactical considerations are simpler, less off-turn actions are likely, etc. I'd say encountering half an encounter as a wave is going to be 2-3x faster in table time to resolve.
Anyway, this is all interesting in an analytical sense, as I really have not done this sort of analysis on my own work. Its always hard to interpret in terms of actual play, but it definitely establishes some baselines at least. From what I understand they did a lot of this kind of thing in 4e development, and it does clearly show!