At a guess I would say you are doing a Thorough scan, I don't know if you are also selecting Archives to be scanned ?
With 4.8 avast is doing a little more on the on-demand scan, the improved anti-spyware and anti-rootkit are integrated into the scan.
I see an increase in the duration of the scan from a little over 11 minutes to 15 and that is for a Standard scan without Archives. So if that is the kind of increase for me when only scanning 8GB of files, I can imagine what it might be for thorough and a lot of data being scanned (?), none of which to mention. Your system is broadly the same as mine no speed demon.
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.