PCs pay a heavy performance tax in the real world that doesn't show up in testing.
When a PC is tested, there's no antiviral software running. Same thing for a Mac.
When I use my PC, I run Norton AntiVirus. My Mac runs bare.
How big is the hit? A backup running in XP is twice as fast with NAV disabled as when it's enabled. Twice. That's one heck of a performance penalty. I have to remember to disable NAV when I'm doing disk intensive tasks. Very annoying.