Hello Vemula,
the SELECT for all entries may issue a full table retrieval in case the internal table is empty. If the snippet captures all relevant code the first query might be intended to populate the internal table. There is however no check of sy-subrc and/or sy-dbcnt after the first query. Maybe adding such an check already solves some of the performance issues you experience currently.
On releated track the Code Inspector (TA SCI) and the ABAP Test Cockpit contains a check "Unsecure use of FOR ALL ENTRIES" that can be used for code scans in order to avoid such a flaw.
Best Regards
Klaus