As per my knowledge if the data base backup is in bulk quantity, you can remove the nsr_parallelism option from lotus configuration file (which is located in client server)
Notes_ExecDirectory = C:\Lotus\Domino
NSR_BACKUP_PATHS = D:\Lotus\Domino\Data
NSR_NOTES_INI_PATH = C:\Lotus\Domino\notes.ini
NSR_SKIPDBERRORS = TRUE
NSR_NOTES_CONNECT_TIMEOUT = 0
PATH = "C:\Lotus\Domino; D:\Lotus\Domino\Data;C:\Program Files\EMC NetWorker\nsr\bin"
as you can see ther is no parallelism parameter present here .Doing like this the database is gonna split into multiple streams as per required and your backup time is gonna reduce.
try it once hope it works