How to resolve low performance issue on PO receipt Page when select PO

we use Peoplesoft SCM 8.8 SP1 from 2006 , and then we have 2M+ records on PO receipt side , Now when user do PO receipt , the select PO page very slow to process , and after the DB level trace , one SQL statement show perofmance very bad

UPDATE PS_PO_RECV_SHIP_T

  SET PROCESS_FLG = 'Y'

WHERE PS_PO_RECV_SHIP_T.SEQ_NBR = :1

   AND PS_PO_RECV_SHIP_T.AMT_ONLY_FLG = 'N'

   AND EXISTS (

SELECT 'X'

  FROM PS_RECV_LN_SHIP_VW

WHERE PS_RECV_LN_SHIP_VW.BUSINESS_UNIT_PO = PS_PO_RECV_SHIP_T.BUSINESS_UNIT_PO

   AND PS_RECV_LN_SHIP_VW.PO_ID = PS_PO_RECV_SHIP_T.PO_ID