Oddly, I've found that different AV software impacts different computers - differently.
On some PC's, Norton products have a real nasty slow-down effect on many processes. On others, not so much - and I mean beyond the mere relation to the overall processing power of the PC.
At this point in time, I am using AVG free and I notice no slowdowns on any processes at all. I've tried PC-cilin, NOD32, Norton on this same PC and they all slowed down my dual-core 2GB rig - and this is considering that only the AV part of the software was running - and I'm talking only the AV piece of the software.
I'm not using my scanner much, so I haven't really noticed anything. However, I would think that the AV software should be "smart" enough to realize that the scanner is part of your internal network and that you can't really obtain a virus through the the image-info. Yes, you might be able to get one that has been embedded in the scanner buffer-memory (very exotic). Hence, the AV software shouldn't really have to scan the incoming bit-stream.
however, if your scanner is on a network - i.e. shared with another PC though eitehr a LAN or a USB hub for example - it's possible that the AV software considers the scanner as an external entity.
Perhaps some AV software has an option to specify what sources should be treated how.