Have either of you done clean installs of the latest avast version ?

If you have two things come to mind:

  1. The Persistent cache would have been cleared and some default settings wouldn’t be populating the Persistent cache. This would have the effect of slowing the scan for the first few scans.
  2. On the good side it could resolve/repair any underlying issues, but it would still slow the scans for the first few scans.

I Generally don’t do on-demand scans, usually as a result of questions in the forums, but I haven’t seen a major difference. However, my systems are relatively light on installed programs and media so any difference would be less than mega GB of data to be scanned.