Backup policy recovering speed slowly (disk)

If job A runs on its own then it completes within 20 hours.

If job B runs on its own then it completes within 20 hours.

If job A plus job B overlap and run together at any time for any reasonable length of time, then one or both of the job A and/or job B then complete within 30 hours - and the overall average speed is reported to be slower (i.e. the reported speed in Activity Monitor of NetBackup Admin Console).

If the above is true, then the reported lower average speed would be entirely normal because the value of speed reported by NetBackup is only ever the average of the total moved by the time elapsed and is not (AFAIK) the actual current speed.  During any backup job there will be times when the backup is actually running faster than the average, and times when it is actually running slower than the average - but the reported average will only change gradually the longer that a backup runs faster or slower.

But also, look at it this way... if job A runs in 20 hours on its own, and job B runs on its own in 20 hours, but if job A and job B start together at the same time, then they both take nearly 40 hours then this tells us that there is at least one bottleneck at some point along the data movement path.

IMO, your only course of action is a thorough review of all elements of infrastructure.  IMO I wouldn't bother tweaking any configuration settings of anything until I understood exactly where the bottleneck is.

e.g. along the entire data path, all the way from source data on source disk, to target backup data on target storage unit, check all of these:

1) If the CPUs of any NetBackup Server, and/or any active NetBackup Client, and/or the CPUs of the Data  Domains, are NOT maxed out during times when both job A and job B are running then that rules out a CPU limitation.

2) then check memory

3) then check disk LUN throughput

4) then check throughput on every SAN interface

5) then check throughput on every NIC interface and bond

.

The thruth is that no finite network system ever has limitless throughput capacity.  i.e. all networks always have at least one bottleneck somewhere.