For backup performance issue, you can isolate things if it is I/O read (from disks) or I/O write (to tape) by running the VBDA CLI.
(examples)
vbda -vol /CONFIGURATION -out nul -profile -log > <OUTPUT_FILE>
vbda -vol /C -out nul -profile -log > <OUTPUT_FILE>
vbda -vol /F -trees "\PROD\SAPDB" -out nul -profile -log > <OUTPUT_FILE>
vbda -vol / -out /dev/null -profile -log > <OUTPUT_FILE>
vbda -vol / -trees "/oradata/PROD" -out /dev/null -profile -log > <OUTPUT_FILE>
It is writing to NULL (nothing).
It will show "Display statistical info" so you can determine I/O read speed.
It is also useful to troubleshoot a backup hang or timeout on one object such as CONFIGURATION.