Yo, Bear...
I don't wanna make you any grumpier than usual, but it ain't a myth.
Independent tests run by PC World, ZD Labs, and dozens of other organizations over the past 5 years (since the release of Norton Antivirus & Systemworks 2001) - not to mention my personal experience, all demonstrate rather conclusively that NAV uses more system resources than nearly every other competing product.
Part of the reason for this is that Symantec/Norton runs so many additional processes: LiveUpdate, SymEvent, and others that run as services under NT-based systems.
I switched to F-Prot more than a year ago, and have been more than pleasantly surprised by its small footprint (2520Kb of memory - 0% CPU) and fast, reliable performance ever since.
Besides... the "0% CPU" measurement is misleading for ANY Antivirus proggie. Under normal operation, antivirus apps are merely resident and watchful. Only when actively scanning (email, downloads, scheduled HD scans, etc.) will you see any noticeable performance hit on the CPU.
All the above BS aside, I was prompted to switch after 6 years of running NAV simply because I don't like Symantec's business model or attitude... and I really hate their decision to use "Product Activation". I ALWAYS paid for my NAV, all nice and legal. But I still dislike the assumption that I am guilty until proven innocent.
And that is what is at the core of Product Activation, whether from Symantec, Microsoft, Intuit, or whoever.
'Nuff said.
Are ya' grumpy yet???
-ManX