When you talk of performance hits on old systems, avast has always tried to get the balance right in the default settings, particularly in regard to performance Vs protection and that is why I feel they would have left that element unchecked.
More so in the Quick Scan as that really is all most should need to do on a regular basic, it is what I do weekly, and that I have also set to Store data about scanned files in the persistent cache and this I'm sure wasn't the case in the default setting.
I normally only run a full system scan when people mention it here that they are experiencing problems and I do that whilst responding to posts.
Dirty archives or clean ones are inert, so why would that be an issue, whilst they should be scanned by the web shield they aren't by the file system shield because they have to be opened, extracted and run and before that happens the executables, etc. would be scanned. Not knowing anything about there testing or settings, I can't comment further and have no intention of wasting any further time on it as it is also diverting the original topic.
Unfortunately there is no Reset Defaults button in avast, something which several of us have been calling for for ages as there are people that are never done tweaking their system to within an inch of its life and haven't a clue what the defaults were manually reset.