Each Vista search slows down

Each search I do with Vista's Explorer search boxes has the peculiar
behavior that, unless it is done very quickly, it slows down. Progress is
typically quite rapid at first, even when a fairly large directory tree
is being searched, but slows to a crawl before long. By the time the
search actually completes, it has often been moving slower than molasses,
or even apparently stalled, for several minutes. Typically 90% of the
progress is made in the first minute or less of a search that takes
several minutes.

Why does the search slow down? I would expect it to proceed at a constant
pace until it was done. XP's search didn't exhibit this behavior, except
if it was looking at the contents of files and hit a particularly large
file. Vista's search slows down like this even for simple filename-only
searches, on the other hand.

Is there a known Vista tweak to make a search maintain its speed all the
way through until it is done?