Well my scan logs don't show any major difference in scan duration. If you did a clean reinstall, your scans would be initially slower as you have removed the persistent cache, which takes a little time to build up. This helps scan speed as files in the persistent cache if they haven't changed aren't scanned.
You also have to ensure that your scan settings elect to a) use the persistent cache and b) gather data for it.
This is the same for both my systems, XP Pro in my signature and my netbook with win7 starter a dual core Atom N550 1.5ghz chip and 2gb RAM. The scan duration is much slower than the desktop as it doesn't have the same grunt.
The GUI loads quickly on both even though the netbook has a low powered Intel chip on the motherboard. I can't do much in the way of comparison with 5.0.677 on the netbook as it wasn't on there that long before installing 5.1.864, but I really didn't see any appreciable difference in any of the avast functions.
