I always run full scan once a week and the latest version of Avast is just so slow. Started scanning 90 minutes ago and it's not even 50% there. On the old version of Avast it took, 1 hour, 90 minutes at the worst
Did you do a clean install ?
Or possibly use some cleanup tool, like CCleaner, etc. ?
1. AvastUI > Scan > Scan for viruses > select the scan you are doing - at the bottom of the screen > Scan Settings > Performance. Scan Priority, Persistent cache & File access.
A clean install will lose the persistent cache, this will have to be rebuilt over a few scans. Not only that you go back to default scan settings (if you made any changes). Such as, not storing data about scanned files in the persistent cache, if you aren't populating the persistent cache, then it doesn't have any data to speed the scan.
2. If using a cleanup tool it could be clearing some avast data files this could impact on the scan if the persistent cache is cleared. Don't let a cleaner clear any avast folders/files unless you know what the impact might be.