I cant understand . "My computer is getting slow. Virus scans take a long time to finish." Is the setting that you choose are not very good? Is it every time that you scan the computer by Avast will detect virus? Its incredible!
I scan my computer yesterday ,about 40GB and avast finish it in 15 minutes
you should set quick scan,and when virus pop up should tick dont show this again(like DavidRs saying)
good luck and enjoy Avast!