CLOB update is very slow

This article is written in oracle9i release 9.2.0.6.0 I just want to share this issue with all...

we are updating CLOB column in one of the table. The update statement is running through Java code. It used to take 1 second to update one row. All of a sudden, it started taking 150 second. Immediatley, developers reported to me.

Here are the direction i took initially.

1. I verified the table statistics. The stat is generating periodically.
2. Enabled the trace and ran the TKPROF. In the execution part, it is using high resources.

update MWS_COLLECTION set WEB_BODY=:1, headline=:2, STATE=:3,
SOURCE_PUBLISHED_DT=:4, author=:5
where
STORY_ID=:6
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 55.99 145.55 1057 9495719 8238 1
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 55.99 145.56 1057 9495719 8238 1
Misses in library cache during parse: 1
Optimizer goal: FIRST_ROWS
Parsing user id: 67
Rows Row Source Operation
------- ---------------------------------------------------
1 update
1 TABLE ACCESS BY INDEX ROWID MWS_COLLECTION
1 INDEX UNIQUE SCAN PK_MWS_COLLECTION (object id 139890)
********************************************************************************

3. I checked the tablespace and it is LMT and SSM is AUTO.

I am not able to find anything except the trace file output shows more resources in exection part.

The LOB segment and other columns use the same tablespace. I thought, i move the LOB segment to different tablespace. But i checked with the business and they agreed to purge 90% of the data in the table. so i backup only 10% of the data into another table and truncate the table and insert the cloned data into original table. The problem is gone and again it takes only one second to update the single row.