Oracle database offline tape backup running slow

Dear All,

We are using SAP ERP 6.0 EHP5 on ORACLE 11g(11.2.0.2)/WIN SERVER 2008R2.

For the past one week we are facing slow offline tape backup issue for oracle database on production server.

Our DB size is 180 GB.

Earlier Production server offline tape backup(scheduled from db13)used to take 1-1:30 hr.(not more than two hours)

For the past one week its' taking 5-6 hours for offline tape backup. It's running in non peak hours(midnight).

Note: I don't find any error or warnings from backup logs or from system logs.

Your valuable suggestions are appreciated.

Thanks

Venkat