I don't have a database the size of yours, but I was experiencing
similar problems with my expriation processing.  In my case I had a
filespace that had 1.6 million files in it and I could never seem to
get expiration to finish for this filespace.  I started it on Saturday
morning and it wouldn't be finished by Monday morning.  In the course
of trying to deal with some other problem, I decided to significantly
increase my Bufpool size.  I increased it to 8192 (8mb) and I saw a
tremendous improvement in expiration processing.  If your database
buffer isn't big enough you'll see excessive I/O busy and wait times,
this is possibly why it's taking so long. A bigger buffer would
alleviate some of that I/O.  I would do a Q DB F=D to see what kind of
cache hit percent that you are getting in the buffer.  I think the
recommendation is that you should see at least a 90% hit ratio.  I
shoot for 95% now that I see the impact it can have. If you already
have your database buffer as high as you can, and your in the target
cache hit ratio range, I don't know what to tell you.  Also, it's
probably a good idea to let expiration all by itself if you can.

Elvin Peterson
S.C. Johnson Wax