I haven't had the chance to try the issue on a super-fast computer. I guess that a very recent system might not be slowed much but how many people run Windows XP on a very fast computer? All my machines are in the geriatric category so I really notice it.
I have witnessed the slowing of Windows Explorer on several machines, none particularly new. Try to scroll across when the monster Windows folder (5.89GB, 27,584 files, 2,606 folders) is open in list view mode. The scrollbar thingy moves very hesitantly.
As I said earlier, my workaround is to use GPSoft's Directory Opus (known affectionately as Dopus) and this is not slowed one bit. It is lovely software (thanks Jonathan Potter) and vastly better file management software than Windows Explorer. I hazard a guess that it would make Windows 7 much more friendly.
There is a genuine issue here and I would be glad if the Avast developers had time to look at it.