Maybe the scanning time depends on the number (and, consequently, the size) of the archive files.
The many archives you have, the long the scanning will be.
Time of on-demmand scanning depends on some other factors (other running programs, resources at your computer, etc.).
Anyway, avast is not a faster on-demmand scanning, on contrary, it could be very very better on this point
Do you want a slow on-demmand scanning? Try ClamWin