Huge Table Insert - take long time?

12.1.0.2 RAC on Exadata with forced logging for standby

There's a refactoring process: (subpartitioned tables with millions to a few billions of rows)

1. rename the main table to main_old
2. create the new_main table (no indexes or constraints at this point)
3. set table prefs for concurrent incremental stats
4. enable parallel dml
5. insert into new_main from main_old using APPEND PARALLEL 16 (note: most tables are compressed)
6. create a PK or an index (as needed) on new_main
7. gather table stats on new_main

The insert in Step 5 took 9.5 hours... Total refactor process time was about 13 hours.

Is there anything you can think of to make this process any faster?

I assume the APPEND hint isn't doing much because the DB has FORCE LOGGING. Guessing alter table nologging wouldn't do much either. Can't exchange from old to new since new mostly likely had columns added.

I've mentioned using alter table if we are only adding a few columns, but the production DBA's are worried about chained-rows, fragmentation, possible explain plan changes, etc. I'm not sure if they are justified in their thoughts.

I've thought of doing the refactor in a diff DB (where I can truly have nologging) and use transportable tablespace, but don't think that would be any better.

Any Thoughts or ideas to try?