Some AV programs use quite a bit of resources. They are running all of the time, I use an AV that only scans if significant file system changes are detected, like installing a new program or downloading files. Under normal use, the real time scanner is in an idle state.
Try tweeking your AV settings, shut off any real time protection, see if that improves the performance. If it does, I'd consider getting a different AV program. I use Eset but only after trying every other one there is, its the one that impacts my performance the least. Mcafee was the worst, Norton second worst. Also, new AV suites overwrite the already adequate Windows Firewall, I don't see any reason to install any of those. Eset makes a Smart Security suite, I only use that in XP systems that don;t have a real full firewall. Norton and Mcafee will put a full firewall in there and it is in a "Learning" state, which means it nags you as you use your machine.
There could be another program that hogs your resources as well, maybe something you weren't aware got installed. Toolbars, Chrome and Firefox Extensions, are all suspect. Look for icons in your tray that you don't know what they are, also change your taskbar settings so it shows all tray items without hiding anything.
Welcome to Windows 8 forums.
