Discover SQL Time on Oracle10g

When you issue an ALTER SYSTEM KILL SESSION, Oracle merely marks the session as killed and does the actual work of killing the session asynchronously. That may take just a couple seconds, it may take many hours if the session has an uncommitted transaction that did a lot of work that now has to be rolled back or if the session needs to stay around in a killed state in order to notify the client that their session was terminated. It may, therefore, require a substantial amount of waiting before a user could be dropped.

You could do something like this where your loop will wait indefinitely for all the sessions to disappear.

DECLARE
  l_cnt integer;
BEGIN
  EXECUTE IMMEDIATE 'alter user abc_user account lock';
  FOR x IN (SELECT *
              FROM v$session
             WHERE username = 'ABC_USER')
  LOOP
    EXECUTE IMMEDIATE 'alter system kill session ''' || x.sid || ',' || x.serial# || '''';
  END LOOP;

  -- Wait for as long as it takes for all the sessions to go away
  LOOP
    SELECT COUNT(*)
      INTO l_cnt
      FROM v$session
     WHERE username = 'ABC_USER';
    EXIT WHEN l_cnt = 0;
    dbms_lock.sleep( 10 );
  END LOOP;

  EXECUTE IMMEDIATE 'drop user abc_user cascade';
END;

Generally, though, I'd question the problem you're trying to solve. I can't think of many times that it would make sense to drop a user that has active sessions at the time you decide to issue the drop. In the vast majority of cases, the presence of active sessions strongly implies that the account should not be dropped.

There are various tricks for some performance improvements in the OCI library(for example chunk size settings). But since you do not have direct access to OCI you should contact you DOA vendor.

If you want to trace user's session from begging you have to create a "LOGON TRIGGER ON SCHEMA" and then call something like dbms_support.start_trace(true) in this trigger. Or you can set tracing in other session using dbms_monitor.

You should first "prove" that the root cause is BLOB handling. Server side log should give you some numbers. (Just beware - it's not easy to read).

PS: there is also a way how you can register your "interceptor" library in OCI.dll. This will give detailed tracing for the client's side and will tell you how your DAO is working. But this really requires some Oracle experience.