Kanga,
Avast is not a simple program, nor is it just "one" program.
Avast is made up of the virus scanner, and 3 resident providers (scanners). It does take a little longer to shut your system down, but only about 15 seconds more since your normal shut down was 10.
This is normal. It is the same with my ME and XP.
Shut down time varies with the number of applications running at the time you command Windows to commence the shut down procedure. Even if you did not have Avast running, but had a few other apps open, your shut down time would be longer than usual.
I am not aware of any anticipated change in future versions.
Good luck and thanks for using Avast!