RMAN backup is runnning very slow

RMAN backup is runnning very slow, average speed is 1MB/sec. How to find out what is making the backup slow.

Around 100 GB DB is taking 14 hours to complete the backup. It was taking only 4-5 hours to complete the Tape backup before, now we have moved this oracle backup to Data Domain ATMOS storage. Not sure why it is running very slow, Data Domain mean it should complete fast right.

Help me to find out what is wrong, that making the backup slow.

Please let me know how to proceed and which are the logs required to bottleneck this issue.