Since you specifically mentioned JPG attachments, I'm just making a guess here.
As you probably know, JPG is essentially a graphics-data compression format. Depending on settings when the file was created, a 60K JPG when viewed would be the equivalent of maybe a 2 meg BMP, right?
I've found that JPG attachments take an unusually long time to download (or send), relative to their file size. My ISP agrees that's because with MIME encoding (the most common form for attachments), they're in effect uncompressed during transmission and then re-compressed into proper JPG format at the receiving end.
I wonder if avast is recognizing these as actually an oddball kind of "archive", and expanding them to be sure they really are graphics? This would of course add even more to the download time. Maybe one of the tech experts could clarify this posssibility?