Mong,
I have been having the same problem. My problem is because of the EndNote X1 add-in. The people from EndNote indicate that, since it is a 32-bit program, it will cause strange behaviors with Word 2007 in a 64-bit OS. I had to completely disable the add-in to get Word to run smoothly in Vista Home Premium 64-bit.
Kind of disappointing as the Cite-While-You-Write feature is one of the best things about the program! Either I lose that ability and have a 'smooth experience', or I keep it enabled and get the perk from the program.
So, your problem, while perhaps not related to EndNote (you might not have it), could be related to an add-in for a program not designed for a 64-bit OS.
My suggestion is to disables your add-ins one at a time to see which one is causing the lag.
I hope that helps.