I can give you an example of what I have seen in the past. I had a Netapp filer that was being shared for SAN and NAS purposes. During backups of our NAS shares, and sometimes during snapshots, the I/O on the disks that were being accessed went beyond what they could handle, and causes the cache to constantly be flushed back to disk. Even though none of the VM datastores, which were on faster disks, were not being heavily accessed, the cache degradation overall on the storage system caused everything to suffer greatly in VMware.
You should probably get in touch with Dell if you haven't already and have them perform some diagnostics on their end to see if there is anything you can change to make this operation more optimal.