Yeah, I know your point. When I used McAfee on my previous machine it was quite noticeable. But my Core2Duo came with eTrust Antivirus from Computer Associates and I don't even notice it at all.
I got a boot sector virus years ago from a friend's floppy disc. Lost almost everything! (Incidentally, McAfee was the only one to get rid of it). Never again did I go without AV protection...