Oracle SQL | How to speed up slow unicode migration of a table with xmltype columns

Recently I have had an issue with slow unicode migration of the database upgraded from 10g to 12.1.0.2. The main problem was a table with xmltype: we spent about 4 hours for this table(~17GB) during test migration, though all other tables (~190GB) migrated just for about 20 minutes.
We used DMU(Database Migration Assistant for Unicode), and the root cause of the problem was update statement generated by DMU for this table:

update  /*+ PARALLEL(A,16)*/ "RRR"."T_XMLDATA" A  set A."SYS_NC00011$" = SYS_OP_CSCONV(A."SYS_NC00011$", 'AL16UTF16')

“SYS_NC00011$” was internal hidden CLOB column used to store XMLTYPE. As you can see DMU added PARALLEL hint, but though oracle can use parallel dml for xmltype since 12.1.0.1, we can’t use it because of its’ limitations:

Changes in Oracle Database 12c Release 1 (12.1.0.1) for Oracle XML DB

Parallel DML Support for XMLType
Support for parallel DML has been improved for XMLType storage model binary XML using SecureFiles LOBs. The performance and scalability have been improved for both CREATE TABLE AS SELECT and INSERT AS SELECT.

Restrictions on Parallel DML

Parallel DML can be done on tables with LOB columns provided the table is partitioned. However, intra-partition parallelism is not supported.

For non-partitioned tables with LOB columns, parallel INSERT operations are supported provided that the LOB columns are declared as SecureFiles LOBs. Parallel UPDATE, DELETE, and MERGE operations on such tables are not supported.

Btw, Oracle didn’t support parallel dml for xmltype on previous releases:

No Parallel DML for XMLType – DML operations on XMLType data are always performed in serial. Parallel DML is not supported for XMLType. (Parallel query and DDL are supported for XMLType.)

So I had to use manual parallelization:
1. Monitor “Convert application tables” step through “View Table Conversion progress” and press “Stop” button during conversion of this table.
2. Create table with ROWIDs of this table and split them into 16 groups:

create table tmp_rids as
select rowid rid, ntile(16)over(order by rowid) grp 
from t_xmldata;

3. Execute

1	`ALTER` `SYSTEM` `SET` `EVENTS` `'22838 TRACE NAME CONTEXT LEVEL 1,FOREVER';`

to avoid “ORA-22839: Direct updates on SYS_NC columns are disallowed”
4. Start 16 sessions and each of them have to update own part:

update t_xmldata A 
set A."SYS_NC00011$" = SYS_OP_CSCONV(A."SYS_NC00011$", 'AL16UTF16') 
where rowid in (select rid from tmp_rids where grp=&grp);
commit;

5. Disable event 22838:

1	`ALTER` `SYSTEM` `SET` `EVENTS` `'22838 TRACE NAME CONTEXT OFF';`

6. Open “View Table Conversion progress” window, click on this table and change “Retry” to “Skip” option for the update step.

This simple method allowed to make unicode migration about 16 times faster.