too much time to delete SnapShot of VM backup

Hello,

We are running Netbackup 7.5.0.6 on a windows server 2008 r2 sp1.

We have a big enviroment of VM machines and all of them are backed up using VMware policies with auto selection.

Recently, I noticed that many VM machine backup jobs finish very fast (30 minutes) but snapshot jobs finish after 8 hours.

Moreover, while the snapshot was deleting, it caused a ping lost to the VM machine.

please advise.