The simple answer is that your data was being written to storage. The line after the 2 hours, 41 minutes says you backed up 1.1 trillion bytes, but bpbkar only had to send 4.5 billion bytes. It took just under 10,000 seconds, which makes about 460,000 bytes per second for the bytes actually transferred.
I don't know why it doesn't transfer faster. It could be your network; it could be deduplication performance. You gave the product version as 7.1.x. I don't remember Accelerator going back that far, but even from 7.5, I think you would find better performance if you upgraded to a current version (if the issue isn't your network).
Besides the bpbkar log on the client, as suggested, you may also find insights from the bptm log on the media server.