I downloaded avast today after not having virus scan software for about a year.
It really would be hard to compare when you haven't been using any AV software for a year.
I'm running my first scan now (thorough) and it's going ridiculously slowly. At first the speed was alright, but now it's taking a very long time to make any progress.
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Other things which will effect the scan duration, your CPU, RAM and amount of data on your HDDs. What other security software do you use ?
Archive (zip, rar, etc.) files are by their nature are inert, you need to extract the files and then you have to run them to be a threat. Long before that happens avast's Standard Shield should have scanned them and before an executable is run that is scanned. Thorough is also by its design very thorough and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate.
I have only ever done a Through Scan with Archives once shortly after installation just to ensure a clean start state, but with XP for example avast will do a boot-time scan after installation if you select it, this I believe will be quicker and reasonably effective. Like everything in life things are a compromise.
Is this normal, and if not what is the problem?
So I think you can see it would be hard to define what is normal as it would change from system to system.