In regards to Backup slowness, there are various factors that needs to be taken into account.
CVPerfMgr, is definitely a log that covers all the basis.
From the logs tha tou have provided, the bottleneck can both be the network between the Client and the MediaAgent (in that case it would be good to do a normal copy and paste between the Client and MediaAgent to confirm the throughput).
If it is the physical write to the Disk Library, you can also try to do a write test from the MediaAgent to the Disk Library to confirm the speed.
The above test are scenario's to rule out any environmental issue.
Hopefully that helps.