How to improve performance using Field symbol ?

Hi Experts,

I have this logic that selects data from faglflexa table and then selects some fields from bsid table. How to improve the performance of these statements using Field symbols?

SELECT  rbukrs racct tsl poper gjahr belnr FROM faglflexa

    INTO CORRESPONDING FIELDS OF TABLE it_tab WHERE

           rbukrs IN s_bukrs AND

           gjahr  IN s_gjahr AND

           poper  IN s_monat AND

           racct  IN s_saknr.

  IF it_tab[] IS NOT INITIAL.

    SELECT kunnr belnr bldat xblnr FROM bsid

       INTO CORRESPONDING FIELDS OF TABLE it_tab2

       FOR ALL ENTRIES IN it_tab WHERE

        bukrs = it_tab-rbukrs AND

        kunnr IN s_kunnr      AND

        gjahr = it_tab-gjahr  AND

        belnr = it_tab-belnr.

    LOOP AT it_tab2 INTO wa_tab1.

      MODIFY it_tab FROM wa_tab1 TRANSPORTING kunnr bldat xblnr WHERE belnr = wa_tab1-belnr.

    ENDLOOP.

    REFRESH: it_tab2.