We are performing windows nas backup on windows 2016 server. data storing on Netbackup appliance 5340 pure disk (Media server) over the network.
Client-side and Media server Netbackup version is 8.1.2
The total size of the backup is 4 TB to complete this initially it takes 45 hrs later it increases to 90 hrs. to speed up the backup performance accelerator even though throughput is fluctuating 6 MB/sec, 28 MB/sec and suddenly it falls down 1 MB/sec.
Network cable, Network switch changed and Nic teaming also enabled still issue persist.
we performed the nbperfchk output has attached.
I agree with @Mike_Gavrilov the problem is unlikely to be network related. Windows NTFS is a notoriously slow file system to backup. Have you measured the speed you can pull data off the drive being backed up? There is an article describing the use of bpbkar wrting to the null device to measure the drive performance: https://www.veritas.com/support/en_US/article.100006447
I also would look at making full use of NetBackup accelerator and enabling Windows change journalling to help improve your backups after the first (change journaling is set under the Client settings for windows clients in the client host properties).