and just so I am not BSing,...
PC World says:
"It works. Impressively well. With games, even. You can see our detailed
test results below, but that's our first impression of Windows XP
running under Apple's Boot Camp
on our 20-inch iMac."
MAC world says:
"As you can see, the Macs running Windows gave these PCs a run for their money"
there is also a graph with numerous benchmark tests showing that macs were rather right on par with the pc's, or actually beat them in some categories... no "slow down" what so ever