Inconsistent scan times

[REDACTED]

The PCs on our network have hardware and software configurations that are highly similar by design.  However, I have found the times required for a full system virus scan by Avast to be quite inconsistent.  Is there a reason for this?

[Eddy]

Each system is different therefore scan times will be different too.
Even a little difference in temperature(s) can make scan times different.

[Para-Noid]

Also frequency of scans, use of the persistent cache, scan settings and how often a file is accessed
are all factors. Once a file is in the cache that file will not be scanned again unless it was accessed.
The file shield will scan the file upon opening. Your full system scan will run it again during the next scan.
Then that file won't be scanned again until it is accessed again.