Playback of an unaltered DV clip in the appropriate DV sequence uses about 15% of my CPU cycles in 2.0, about 30% in CS3, and well over 50% in CS4. I'd kind of like to know myself what the hell has changed under the hood that such a simple activity as DV playback requires so much in the way of resources.
You're running an i7. I'm using a P4. I would expect to see your percentage much lower.
The question really is why such a big difference between versions? What changed?
At least one of the PPro plug in software companies have noticed this as well, see it as a problem, & have not recieved definitive answers from Adobe yet.
Apparently it's for real, and not some sort of system specific glitch.
On my i7 Vista64, simple playback of EX1 mp4 footage runs the cpu @ 22-25%, mostly on only 2 of the 8 cores