Performance issues - SE16n VS. ABAP SELECT

Hi Experts,

We're selecting from table FAGLFLEXA in SE16n vs. SELECT statement in ABAP standard code with same parameters, yet SE16n takes only a few seconds while the SELECT statement in function module G_RWIN_REVERSE takes more than 30 minutes until it times out. There are no custom indexes created for this table. Here is the code in the FM:

        SELECT * FROM (ledger-sitab) INTO CORRESPONDING FIELDS OF TABLE lt_glu1
          WHERE belnr  = ls_bkpf-belnr AND
                rbukrs = ls_bkpf-bukrs AND
                gjahr  = ls_bkpf-gjahr AND
                  rldnr = ledger-rldnr.

Questions:

1. Why is there a difference? And why so significant?