I'm rather sceptic about such kind of "comparison" - in my opinion, the results will heavily depend on the settings of the antivirus (what files it scans, whether it scans the files on reading/writing/execution, whether it scans the content of archives etc.) - much more than what antivirus it actually is.
OK, for some antiviruses, you may not have the possibility to change the settings... but with avast! you certainly have - and the result would vary according to the particular settings.
Oh, btw - I'm not sure if it has much to do with CPU performance either - the disk access may be the important aspect here.