As Igor said, those files are large, rather to say huge... I use Norton Ghost for making image files of my partitions. For example, my C: partition which is btw, 35.6 Gb with 5.7 Gb used space, gives me 2.1 Gb image file (GHO) plus 825 Mb (GHS - spanned image file). So in general, two very large files, so I guess it's normal that avast! need to take some time to scan everything...
And again, as Igor mentioned above, putting folders in exclusion list should work fine.
Cheers !