"make delete-old" performance.

I recently ran "make delete-old" on a -current box and felt it was
rather slow.  That prompted me to do some more careful experiments.

On one box where I have both 8-stable and 9-stable available, there
was a ~30x slowdown (based on 5 runs, ignoring the first).  I don't
have a -current world on that box so I can't directly compare but on
another pair of fairly similar boxes, I get a ~180x slowdown between
8-stable and -current (and that figure is probably optimistic since
the -current box was idle whereas the 8-stable box was fairly busy).

I realise that "make delete-old" isn't something you nede to do every
day but going from sub-second to multi-minute duration is quite
noticable.  Can anyone suggest what has caused the change?

-- 
Peter Jeremy