Are you sure it is the COBOL performing poorly? Perhaps it is the prepared statements that COBOL is calling. Poor performance at database level impacts the length of time the COBOL process takes to complete. What performance testing steps have you already tried in order to isolate the problem?
Did you ever figure this out? We're experiencing a similar phenomena following either an Oracle 12c upgrade or Tax Update 17B.. trying to figure it out at the moment. Any help would be greatly appreciated.