I got the same problem in Vista (since the beta).
As you know, ActiveSync no longer works in Vista. When I was in the era of XP, ActiveSync works fine and doesn't cause any slow down problem to my PC. (But the WM device still syncs like hanging there) Since the very first version of Device Center to the current version, it's a disease to my PC. Currently, there are no solutions to this problem.
Since I'm now using Exchange connnections, I just disabled the Device Center. This works fine for users like me: plugging the USB for charging only.