Well, that is a very open-ended question, so here are some high-level thoughts on the matter.
First off, you just migrated from a rather old version of DB2. Version 4 went GA in September 1994 -- that is about nine years ago! You ought to consider bringing your subsystem up to a more recent version of DB2 -- at least V6. That way you can be more sure that your system is taking advantage of more recent performance techniques.
Secondly, whenever you move to a new version of DB2 you should consider rebinding your application programs. New versions typically add optimization features that your programs cannot take advantage of unless they were rebound. Before rebinding, consider running RUNSTATS on your DB2 objects to get the most up-to-date statistics in the catalog. This will ensure that DB2 is formulating access paths for the SQL in your programs based on the most current information about your data.
My third, and final thought on the matter is to ask, "What else changed?" If the answer is honestly and truly nothing, then your only recourse is to REBIND (and/or migrate to a newer version of DB2). If something else also changed, anything else, then you should take a look at that as another possible mitigating factor in your performance degradation. For example, did any of these "things" change:
Make sure that none of these changes could have a negative effect on performance. And good luck!
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