Since the recent update to version 18.1.2326, the scans still take several hours but the scan history report shows that every scan is lasting only "a few seconds".
Same situation here - Full Scan runs & reports 4:18 hours.
I believe this has been acknowledged by an Avast Team member in another topic.
I tested this using the Quick Scan and that too reported Run time: a few seconds not an actual time.
Aside from that the Quick Scan was positively glacial, nothing like the original Quick Scan which actually did take just a few seconds. Avast have made a pigs ear of the reintroduction of the Quick Scan it is anything but quick, reporting it took 'a few seconds' is a joke and a bad one at that.
I started off a Smart Scan to compare, only to find that after the latest program update, Sensitive Data Scan was added to the Smart Scan. This is bad considering, A) Sensitive Data protection is a paid option and B) I didn't elect to install it. Just a bloody hook from marketing gremlins.
Anyway on with the Smart Scan comparison to the Quick Scan (I removed Sensitive Data protection option before running the smart scan):
1. It completed much, much quicker than the Quick Scan.
2. On successful completion, I checked the scan history only to find that the Smart Scan I had just run isn't recorded.
Is it any wonder that I don't bother with on-demand scans outside of testing.