I think you have issues with the 3x BL860c because of reduced read performance from disk subsystem. Can be anything from kernel parameters, nMP, volume manager or filesystem options. The backup performance gets worse, since the throughput is not able to saturate the LTO4 drive. Once the drive is in start/stop mode, you will see this kind of additional performance drop. Fix the read performance on the BL860c to resolve your backup issue. You can use
/opt/omni/lbin/vbda -name DATALIST -vol / -trees /etc -profile -report -out /dev/null to test local filesystem performance without using the network at all.
Regards,
Sebastian