Job Rate steadily dropping

We started a large job off 4 days ago and the job rate was just under 9,000 MB/min when it started. Yesterday when I checked the rate was around 3,200 and today it's at 2,762.

What could be causing the slow decline in performance?

Our server has a 4 Gb/s NIC team which is trunked to the core switch. The HBA is a 6 Gb/s card and the tape and drive are LTO-7 standard.

The data being backed up is 8 TB of mostly audio and video files.