Most likely because you have on the file monitor, which scans files as you open them. And doing a scan while using the computer doesn't surprise me that it's going slow, especially if you have a lot of other stuff going on at the same time...
I just have Nod32 though, and it's never slow when I use my comp...even while it scans from a schedule, it does it in the bg and I can use my comp like normal.