SAS doesn't scan files greater than 4MB, so you really can't compare the two scans.
On my system a Quick SAS only scans 7000 or so files, where a standard scan without archives (if you are enabling archives, I wouldn't bother they are inert) scans 4 times as many files and I don't have a huge amount of data.
You have never clearly stated what type of scan it is that you are doing, Thorough, with archives ?
Thorough is also by its design very thorough (it scans all files) and perhaps a little overkill for routine use, were a Standard scan without archives should be adequate. 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.
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.
In the window of the scan it shows the last file scanned, that should give a reasonable idea of the folder it is in and the likely next file if it follows the same sort of order as explorer.