Hi,
It is pretty much expected behavior.
There are a couple of things you can do to improve throughput.
Disable atime, so that you don't update meta data on reads. Usually you don't need that feature and it does make a difference.
Disable sync.
vSphere wants to aggressively sync when writing data. While that is a good thing for if you run a VM of that disk, it might be less interesting if you are using it for backup purposes.
The following article is for FreeNAS, but depending on your storage implementation it might be relevant there too.
IOW, don't disable sync writes if you intend to run VMs from that datastore.
If you're not using ZFS then look into the NFS async mount option. The same warning as above counts of course.
--
Wil