I was thinking it could be the freehub also, but mine never made any noise or significant drag until I was coasting above 30 mph for enough time to heat it up. LBS owner couldn't replicate it on the flats no matter how fast he got the bike, since he couldn't coast long enough to heat up (and therefore he wouldn't believe me... but that's another story

).
Still, that wouldn't affect you unless you're freewheeling. If it feels slow when the pawls are engaged - when you're pedaling fast enough to keep up with the wheel - it isn't the freehub.
Loose spokes - or just a floppier wheel overall? I had no idea how flexy spoked wheels are, and how much power you lose there, until the first time I rode a disc.