Invoice Billing Process

I posted this question yesterday but got no hits, so I'm sending again in
hopes that yesterday's message failed to reach the entire list.  The list
is usually great about offering solutions to our beloved BPCS applications.

BPCS 6.1.01 Mixmode - AS400

We currently process around 800 invoices each night with our central
billing run of BIL500.   The processing time for this job has recently
grown to about 1 hr. and 40 minutes.  I can't remember when the performance
slowed on this job but we did recently implement V4R5.  We did however
apply all recommended PTF's etc per the BPCS list.
STRDBMON against the BIL501B job shows some high index counts on a couple
of files but does not recommend any logicals to build.  I suspect we need
to build something but don't have a clue what to build.  We are also
looking that STRDBG on this job to see if we can determine where the
performance hit is.

Any ideas on what we can do to improve performance on this job.
How accurate is DBMON on these jobs when it illustrates a high index count
but shows no logical fields to build.
Any suggestions outside of buying some bolton software to make this work
better.