Oracle 11G - Update Is Very Slow On View?

I have big trouble with some Update query on Oracle 11G.I have a set of tables (5) of identical structures and a view that consists in an UNION ALL of the 5 tables.None of this table contains more than 20 000 rows.Let's call the view V_INTE_NE. Each of the basic table has a PRIMARY KEY defined on 3 NUMBERS(10,0) -> INTE_REF / NE_REF / INSTANCE.

Now, I get 6 rows in another table and I want to update my view from the data of this small table (let's call it SMALL). This table has the 3 columns INTE_REF / NE_REF / INSTANCE.

When I try to join the two tables :

SELECT * FROM T_INTE_NE T2
WHERE EXISTS ( SELECT 1 FROM SMALL T1 WHERE T2.INTE_REF = T1.INTEREF AND T2.NE_REF = T1.NEREF AND T2.INTE_INST = T1.INSTANCE )

I get the 6 lines in 0.037 seconds

When I try to update the view (I have an INSTEAD OF trigger that does nothing (just return for testing even without modifying anything), I execute the following query :

UPDATE T_INTE_NE T2
SET INTE_STATE = -11 WHERE
EXISTS ( SELECT 1 FROM SMALL T1 WHERE T2.INTE_REF = T1.INTEREF AND T2.NE_REF = T1.NEREF AND T2.INTE_INST = T1.INSTANCE )
The 6 rows are updated (at least TRIGGER is called) in 20 seconds.
However, in the execution plan, I can't see where Oracle takes time to achieve the query :
Plan hash value: 907176690

[code]....

Predicate Information (identified by operation id):

2 - access("T2"."INTE_REF"="T1"."INTEREF" AND "T2"."NE_REF"="T1"."NEREF" AND
"T2"."INTE_INST"="T1"."INSTANCE")

Note- dynamic sampling used for this statement (level=2)

Statistics
-----------------------------------------------------------
3 user calls
0 physical read total bytes
0 physical write total bytes
0 spare statistic 3
0 commit cleanout failures: cannot pin

[code]....

I get exactly the same execution plan (when autotrace is ON).Furthermore, if I try to do the same update on each of the basic tables, I get the rows updated instantaneously.