I thought that's what OS are *supposed* to do. Run programs. Our job... to know, or at least enlist the help of antivirus software, to know which programs are safe and which are not.
We're using Avast, and I've seen no performance loss with it, but I've used others including Macafee that sure did.
Errr.. Avira. Not sure why it came out of my fingers as Avast.. ;~)