Constant source bottleneck; vSAN

We use a small hybrid vSAN cluster in our environment for production VM's, and one of them is our dedicated Veeam server.

The Veeam server was just recently built, and as it runs on our VM cluster; it should have enough resources to backup quickly.

Unfortunately we found that the backup job bottleneck always seems to be the source... Which is a flash backed vSAN array. The backup target is a NAS that is connected to our vSphere cluster with iSCSI, and a VMDK provisioned for the the Veeam server on the LUN.

As the VMDK is presented to the Veeam server as a local disk; we formatted it as ReFS with 64KB blocks in Windows.

We are testing a small portable hard disk as a backup copy target from the NAS, and it STILL shows the source as the bottleneck. I am not sure what is going on. The backup copy is slower, which would make me think it was really the target. I do not see any CPU or RAM excess utilizations on the Veeam server's OS.

Any thoughts on increasing performance?