Table(s) grown to 10 million rows, SQL performance unacceptable to customer

Oracle Enterprise Version 11.2.0.4.0 RAC No Platform HP-UX IA (64-bit)

Legacy SQL statement, executed by scheduler task once a second. Tables been left to grow SQL Query now takes 1.25 minutes.

Customer at this stage not wanting to lose data therefore did CREATE TABLE xyz as SELECT * FROM source for back up purposes. But reimporting back up data will be troublesome

On TEST db can TRUNCATE source tables and SQL executes 00.01 seconds.

Have ran SQL through SQL Developer tuning guide , profile 15% improvement or remove expensive UNION-ALL. Stats been refreshed ,

Wonder if any ideas please? Don't know if large scale DELETE's of processed rows would improve , still full table scan on empty blocks I understand.