One possibility is your Avast may be configured to scan all archives, so Avast is opening them all and scanning their contents.
Another possibility is that some area of your HDD has some problems (whether physical or filesystem issues).
To be clear, I am just throwing out possible reasons, and I have no indication if any of those are actually your case.
You may want to review your settings for the specific scan, or change the range (instead of, say, the complete drive, divide you scan areas and ease some scan settings). Or running at night some scandisk, or even some HDD manufacturer's tool.
The suggestions are related to the possible reasons I wrote about, so I can't be more specific. They may or may not help you, so I would start by the simpler methods first.