I've run across a side effect of SO-A checking for duplicate PO Numbers.
 
If a relatively general use PO Number like "Literature" or "Exchange" is used, there is a significant delay (up to 5 to 30 seconds) while information from other sales orders spins by on the screen while checking for duplications.
 
My first thought is to make these informational type PO Numbers be more specific by including the Customer Code.  For example "Exchange" would become "DS21 Exchange".  This would work fine for us.
I would be interested in how others may have solved or will solve this problem.
 
A SO-A program problem that I think may need to be addresed is related to arrowing up though a Sales Order header.  When You arrow up through the PO Field, the Search for a duplicate commences.  If possible, it would be better to only do this when the PO field is entered or edited and  the return or down arrow is pressed.

You are correct.  However, it appears that the algorithm used to look for duplications is following the PO key in the BKARINV table.
 
The TAS application then needs to read each record with that matching PO to determine if there is a record where the Customer Code matches the customer code on the new order being entered.  I just counted and I have 208 records in BKARINV that have the PO # "Literature".  How many Orders out of the 1000 do you have that have an Identical PO??  It's these 208 records that it takes time to step through.
 
This is a perfectly acceptable way if you can't request a record directly from the Transactional engine that matches a given PO # and Customer Code.  The Transactional API has a function (#36) that returns a record following the current index and apply a filtering condition.  This forces the Transactional engine to only return relevent records.  The TAS interface to the Transactional API must not give programers access to these Extended functions.
 
There are many reports in DBA that could benefit by being rewritten to step through a index instead of just reading the entire file and only keeping the records of interest.