PO approve performance poor caused by following script

Checked following script showing poor perfromance. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

 SELECT DISTINCT STATUS_CODE, RESULT_CODE, AUTOMATIC_ENCUMBRANCE_FLAG,
  SOURCE_DISTRIBUTION_ID_NUM_1, SOURCE_DISTRIBUTION_TYPE, TRANSACTION_AMOUNT,
  REFERENCE15
FROM
 ( SELECT GLBC.STATUS_CODE STATUS_CODE, GLBC.RESULT_CODE RESULT_CODE,
  NVL(GLBC.AUTOMATIC_ENCUMBRANCE_FLAG, 'Y') AUTOMATIC_ENCUMBRANCE_FLAG,
  GLBC.SOURCE_DISTRIBUTION_ID_NUM_1 SOURCE_DISTRIBUTION_ID_NUM_1,
  GLBC.SOURCE_DISTRIBUTION_TYPE SOURCE_DISTRIBUTION_TYPE,
  PO_ENCUMBRANCE_POSTPROCESSING.GET_SIGN_FOR_AMOUNT(PBD.EVENT_TYPE_CODE,
  PBD.MAIN_OR_BACKING_CODE, PBD.ADJUSTMENT_STATUS, PBD.DISTRIBUTION_TYPE) *
  PBD.ACCOUNTED_AMT TRANSACTION_AMOUNT, PBD.REFERENCE15 REFERENCE15 FROM
  XLA_DISTRIBUTION_LINKS XLD, PO_BC_DISTRIBUTIONS PBD, GL_BC_PACKETS GLBC