DB2 CASE statement faster with parentheses around entire WHEN
In situations where one static sql (procedure) executes better than an equivalent one, I would examine:

a) whether this depends on the order of execution, i.e. does the second one benefit from the first one reading data from disk into the bufferpool? If one runs better than the other regardless of the order they are run in this is not the case.

b) are the plans equal? If one procedure is compiled with one set of statistics, and the other compiled with another set of statistics this could result in different plans. Try rebinding (or recreating) both procedures with the same statistics and see if they still differ