Evidence is that HD overworks with Avast Standard provider on Vista. It's not simply a matter of "speed" (that may vary depending on your powerful hardware and you might not notice). You have to watch and hear the HD.
With Avast running every action forces HD loadings, without it Superfetch of Vista works very well loading just a little bit from HD (and is also faster obviously).
The fact that's not been widely reported is beacuse:
1. Vista + Avast is not so frequent because a lot still have XP...
2. if you don't look and hear to your HD you won't "notice" the problem and will just say "Well, it's Vista that is slow, and the AV has a reasonable impact over it", that is wrong: it has a huge impact, but you don't mind at it and won't blame Avast but Vista which is new etc. etc.
You have to make the speed test with big applications or simply running Administrators tool in Vista (Event Viewer and Scheduled tasks for example). They will make HD overwork, always, instead of using Superfetch and loading faster and without HD noise at all (almost).