When you drop the table on which the stored procedure (SP) depends, then DB2 notices that. The next time the SP is called, DB2 must verify that all dependencies can be resolved. Due to the DROP TABLE, this is not the case, of course. So DB2 has to recompile the stored procedure on the fly. This takes some time.
Do you see the performance degradation also when calling the SP repeatedly - without dropping anything in between?