Hey Mac !
Yes it feels a bit sluggish immediatly after install but give it a few days and it'll be back to it's responsivness. Quote from MS:
The Windows Vista SP1 install process clears the user-specific data that is used by Windows to optimize performance, which may make the system feel less responsive immediately after install. As the customer uses their SP1 PC, the system will be retrained over the course of a few hours or days and will return to the previous level of responsiveness.
BTW how long did it take to complete the install on your p4 system ? It was done in just under 30 minutes on my machine ...