Ah now we get to the real question, comparison, I don't know what Norman scanned or its sensitivity so it is almost impossible to draw a direct comparison.
Right there is a beta build (4.8.1178) currently which is addressing a slowdown in the on-access scan from the update from 4.7.1098 to 4.8.1169 I say my on-demand scans increase and there has been a lengthy debate on it in the forums.
That now seems to have been resolved in the current beta build, where my on-access scans are now almost the same as the 4.7 ones, there is a little more going on in the 4.8 scans in the form of an anti-rootkit scan. So that will add a little to the overhead but will also add to the overall protection, it is a balance.
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.