Hi David,
I guess it might be hard if this is std code all along that takes that much time to process operation you describe.
You might try some to do some things to speed this up if you are sure that reading VBUP is taking most of the time:
1) add an index on some fields of this table (this might not be needed as the logic should know the key which is sales order no and item no
2) try to archive some old data that will free the VBUP from some records and will speed up reading
Of course you might also ask SAP via OSS channel but I guess if it's only a minute they won't really treat it as any bug and will answer "work as designed" and suggest creating smaller sales order (split the big ones).
Good luck anyway...
Tomek