In our case the configuration is set at 64 KB, which has never changed, and the verify files is turned off. The backups still take too long. The only thing that has changed was the upgrade from 11D to 12.5.
We are having the same problem, except even slower. Our current backup runs about 700 GB (4 Tapes on HP Ultrium II) and runs over a dedicated network. On 11 D, this would take 8-9 hours. Since upgrading to 12.5, the exact same backup, takes 6 hours f...