delay changing sort order in explorer on vista

hi,

I have a folder with about 40 documents in it, and I am viewing it in
explorer

When I click on the column headings to change the sort order, a
progress bar appears at the top and it takes 30-40 seconds to
redisplay with the correct order.

this happens on most but not all folders. the delay seems to be about
the same regardless of the number or files/folders in the selected
folder, and while it is doing this there doesn't seem to be much cpu
or disk activity

does anyone know what it's actually doing and how i can make it work
sensibly

Andy