Whilst I don't use Avast One.
The first scan usually takes longer as it does gather data to make subsequent scans quicker (depending on your settings) an example of some of mine from Avast Free (not One) in the attached image.
Now I have no idea if Avast One has options for the scan sensitivity, etc. (or where it might be in the UI).
That said in an on-access antivirus scanner, on-demand scanners are much depreciated as any active file is going to be scanned by Avast before it is created/executed/modified, etc.
