Significantly slow performance when selecting from sys.aud$ in production vs. from test instance.

I've just noticed in TEST selecting from sys.aud$ ... performance is significantly better than same select  issued in Production.     They  both have a large numjber of rows (3 months retention).     Test takes a couple seconds, production takes 30-45 seconds.

This is a RHEL 6 server, db has been relatively recently upgraded to Oracle 12.2.0.1

I've read a lot of articles, not found the cause yet.

Yes, both are in the SYSTEM tspace, and that will be remedied shortly by moving to a new AUDIT tspace.

My plan if I can't resolve will be to truncate the table and move it to the new space and watch it more closely.