Though you've already resolved the primary issue of Quick Scan vs. Full Scan
and identified some potential reasons for the different results on different
systems, the core issue is really rather simple. It's based on the balance
between processor and disk system performance.
Since some files like TIFF or BMP files are large but not packed, they take
more time to read from disk than to scan, so they generally cause more disk
activity and less processor utilization. On the other hand a ZIP file will
generally cause less disk activity to read since it's compressed, but much
more processor utilization as the files it contains are unpacked and scanned
in memory. If the processor in a particular system is slow (single core, low
number), but the disk system is high performance, the scanning process will
tend to saturate the processor much of the time resulting in a high Processor
Utilization number.
The interesting thing is that most people will focus on Processor
Utilization as the source of sluggishness, since they can easily see this in
Task Manager or other system monitor applications. However, the real cause of
slugishness is usually more dependent on the disk subsystem, since the
processor can be easily interrupted to perform a different task. Generally,
the disk system must complete it's requests more sequentially, so if a large
file read has been requested it will need to complete before moving on to the
new request made by a user. This is the momentum like 'grab' effect that's
most often noticed by the user during a scan.
Since older systems tend to have both less powerful processors and less
efficient disk subsystems, they take longer overall to perform the same scan,
even with less total files to scan. However, since it's more difficult to
monitor the activity of the disk sub-system in a meaningful way, it's roll is
rarely recognized and the processor receives all of the blame. Though there's
still a dependancy on the antimalware application's design, the differences
seen using the same application on different systems are more related to
these hardware associated effects.
Bitman
