There could be many causes of slow tape backup, like: faulty cabling on the tape drive, old driver/firmware, other hardware issue, etc.
But if all hardware/driver issues have been ruled out, then there could be another factor which can cause slow tape backup, like the following ones...

It could be a leftover TraceLogLevel Dword value in the Registry, under HKLM\Software\Microsoft\Microsoft Data Protection Manager.
DPMRA is responsible for writing data only to dpmracurr.errlog during the tape write, but if you have trace log enabled then DPMRA is going to write data to a lot of other files which will eventually cause a delay in writing data to tape.
So you need to check that at first, if such a registry entry exists on your DPM server.

If not, then you will need to check the following, in the given order preferably:

Are there any other backups/recovery jobs running at the same tape backup time? Disable them if exist on the targeted server/s and test the behavior again.

You need to set Exclusions to your Antivirus software. You need to exclude the DPM Directory and agents  from  Antivirus scanning. More information here:
Configuring Antivirus Software

(Note from the article:

Configuring Real-Time Monitoring for Viruses

To minimize performance degradation on the DPM server, disable antivirus real-time monitoring of replicas for all protected data sources by disabling real-time monitoring of the DPM process DPMRA.exe, which is located in the folder
Microsoft Data Protection Manager\DPM\bin. Real-time monitoring of replicas degrades performance because it causes the antivirus software to scan all affected files each time DPM applies changes to the replicas.

Additionally, if you experience degraded performance while using DPM Administrator Console, disable real-time monitoring of the csc.exe process, which is located in the folder Windows\\Framework\v2.0.50727. The csc.exe process is the
C# compiler. Real-time monitoring of the csc.exe process can degrade performance because it causes the antivirus software to scan files that the csc.exe process emits when generating XML messages.

For instructions for configuring real-time monitoring for individual processes, see your antivirus product documentation. )

Another option would be to dd the following registry key, on your DPM server:

Data Protection Manager\Agent]
Once you add this registry key and restart the DPMRA service, you will need to check the issue again.

Here also you may get additional information on this:

This issue could also be related to: 

TCP Chimney:
To disable the TCP Chimney you could follow the below information: or this:

Another Backup Solution in place

NIC Driver update.
Please check, when exactly you updated the NIC driver and apply the latest version.

 Finally, Remapping Tape Drives, could help.
More information here:   or here: