It could possibly be a AV issue.
I havent used Avast since back in the XP days myself. As such, I'm unsure if it wants to scan everything when opened, even if its been scanned recently. Perhaps others will know more about it.
You will of course, get many opinions on which Av is best

, but my thoughts for what its worth would be:
Have you tried MSE?
It is a highly recommended, light weight, free alternative with an excellent reputation around these forums.
If looking for a paid software with more features, I would recommend NIS2010.
Some downloads may have a slight pause before releasing it to the download folder. (Especially certain .exes) if the initial scan decides to look into it further. or at least my norton will on occasion.
But 30sec. seems a bit excessive.