There is no separate performance tuning guide for BigData backups, so, the normal performance tuning steps will apply, like identifying the bottleneck, buffer tuning, etc.
Starting point would be to check bptm on the media server to see where the delay is.
At the start of the job, all buffer info will be logged.
At the end of the job, 'delays' will be logged. Look for something like 'waited for full buffer ....'
This extract from the manual has a list of other logs that may be useful: https://www.veritas.com/content/support/en_US/doc/127664414-127666185-0/v127786066-127666185
nbaapidiscv
bpbkar
bpbrm
