Symantec BE15 backup files on NAS performance is very low

we are using BE 15, installed on physics windows 2012R2 server, we have a volume on NAS linked via iscsi to a VM on vmware host, agent was installed on VM server, but when backup files on that volume, the performance was very low, only about 30~40MB/min, what's the problem, which we need change?

Thanks