3 hosts in the Hyper-V 2012 cluster and few CSV volumes.
The slow backup performance (around 1xxx MB/min in backup exec) will occure when it meets the following scenario :
1. VM on Node 1 and CSV 1
2. CSV 1 owner is Node 2
When i change the owner of CSV 1 to Node 1, the backup performance will return to normal (over 6000MB/min in Backup exec).