Can you describe your setup more?
Things like, if your Veeam server a physical server or a VM, if physical is it SAN connected and if not do you have a VM that's setup as a Veeam proxy?
Also, what is listed at the bottleneck when the job completes?
Initial guess is you have a physical Veeam server and no SAN connection to it, and also no VM based proxy server setup so it's using NBD mode to do the backup. The simple solution is to either create a new VM and install the Veeam Proxy role on it (and remove it from the server) or connect the Veeam server directly to the SAN