PF Usage bar tells you how much ram is being used. If that number is close to the amount of physical ram installed, or over, then you are hitting the slow road.
E.g. I've got 1 Gig installed. Current usage says 650 MB. Still got 350 MB spare so I'm OK.
In the Processes tab you can see exactly how much each app is using and the AV software should be listed.
If it's not ram then it might be that all the real-time monitoring is overloading the processor and slowing down your other apps. If it's that then consider de-selecting some of the monitoring options of the AV software.
