The first time I noticed this on the 17'th of Feb a number of issues were reported of which I'm certain they were all false positives. However I didn't use the files affected anymore so I decided to delete them just to make sure. Here's a screenshot of that scan but a bit hard to see due to the bad 4K scaling in Avast.
Today I had an issue reported as well and not sure if it was a false positive or not. The file triggering the alert was a file in my recycle bin called $ROM093N.adi where the threat reported was Win32:Hupigon-ONX [Trj]. I decided to move the file to the chest thinking I might want to investigate it further and it looked like Avast! was able to successfully carry out the action I asked for but when I later checked my virus chest it was empty?!
I also find the new GUI to be a bit confusing not really letting me know when an action has been completed...in this case moving the suspected file to the virus chest. Instead I had to manually press the 'Back' button and I could only assume the action had completed successfully.
I then decided to restart my computer and do another full virus scan and this second scan came out blank with no reported issues and it took just as long as the first one just under 2 hours vs the normal just under 1 hour.
I'm pretty sure this started with the latest version of Avast and the new GUI. Never saw this issue with the previous version(s) of Avast!
Here's another guy reporting very slow scans recently ->
https://forum.avast.com/index.php?topic=197802.0Sorry, forget to comment on settings used for the full scan. Nothing changed in there as far as I can remember. What I did try was to enable all settings found under the performance section on the settings page and I also changed the prio to high but all to no avail I'm afraid.