DD880 slow NFS read performance

I have two Data Domain 880s which my company uses for NFS storage of files that are not accessed often.  We are attempting to move the data to a Dell NX3240 Windows Storage Server.  These servers are connected using a 10GB fiber network.  Moving files from the first system went OK, the biggest problem was that the machine rsync'ing the files would have issues because the Windows server could not keep up.  Now we are moving data on the second system and the copy is going far more slowly, about 10% the speed we saw with the first Data Domain.  I am not an expert on Data Domains and I have no idea why we are seeing such poor performance.  They are both on the same network.  We are not seeing a lot of network errors.  We use virtually the same command to mount the disks on the machine doing the rsync.  What do I look for troubleshoot this issue.  Both of the Data Domain systems are no longer under support and we can not even get support from Dell because the systems are End of Service Life.  I have about 50 TB to move, and at the speed it is moving we will be at this for months.

I also connected a couple of other Linux systems to the Data Domain and had them copy files to there local disk, just to take the Windows system out of the equation.  They are also seeing slow performance.  When backing up from the first system they do quite well, from the second system they see 12-15% of the speed they saw when copying from the first system.  Just trying to get this data moved before I retire.