Delete statement is very slow !!

Can someone explian me why this delete statement is very slow ?? any suggestions !!! pl find the details below !!

Thanks
Madhu

Statement:
++++++++
delete from <tabke_name> where eow_date<ADD_MONTHS(TO_DATE(:b0,'YYYYMMDD'),((-1)* :b1));

PLAN:
+++++

Plan Table

| Operation                 |  Name    |  Rows | Bytes|  Cost  | Pstart| Pstop |


--------------------------------------------------------------------------------

| DELETE STATEMENT          |          |     1M|   20M|  66106 |       |       |
|  DELETE                   |WIN_STORE |       |      |        |       |       |
|   TABLE ACCESS BY GLOBAL I|WIN_STORE |     1M|   20M|  66106 | ROWID | ROW L |
|    INDEX RANGE SCAN       |WIN_STORE |     1M|      |   2616 |       |       |


--------------------------------------------------------------------------------

Session_IO:
+++++++++

11:27:49 SQL> @session_top_sql

OS User                        DB User                         SID # of UndoBlk     LOG IO     PHY IO HASH VALUE SQL ADDRESS
------------------------------ ------------------------------ ---- ------------ ---------- ---------- ---------- ----------------
batchusr                       RTKBATCH                         77 52349           6223101     987505 1253119663 C0000000BB3BC2B8

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