I spoke too soon. While this worked fine when there were no indexes and finished within 10 minutes, with GIN index on the jsonb column, it is taking hours and still not completing.
It is always recommended to create indexes AFTER loading data. Sometimes it can be faster to drop all indexes on the table, load huge data, and re-create the indexes but there's no hard & fast rule. If you are adding 100k records to an empty or near empty table, I would remove all indexes and create them after. Be sure you have sufficient maintenance_work_mem also.