SQL & PL/SQL

Hi,

I ran an update on 1,00,000 rows yesterday.
it took 3 min to run this update.
Today the same update is taking 1.5 min to run on 1,000 rows.Can somebody please help me.

 UPDATE (SELECT *
                FROM rating_temp
               WHERE calltype = 0 and connectedcallingnumber like '91%') ndrt
         SET ndrt.calledcallzone =
                (SELECT callzone
                   FROM callzone
                  WHERE SUBSTR (ndrt.connectedcallingnumber, 3, 3) =
                                                                  callzone.ndc)
       WHERE EXISTS (
                SELECT callzone
                  FROM callzone
                 WHERE SUBSTR (ndrt.connectedcallingnumber, 3, 3) =
                                                                  callzone.ndc);