Backup to an SMBv3 network share or replication from one network share to another one is slow.
Multichannelling causes performance degradation.
SMB Multichannel is enabled by default. Disable SMB Multichannel either on the client, or on the server side by using the following PowerShell cmdlets:
On the SMB server side:
Set-SmbServerConfiguration -EnableMultiChannel $false
On the SMB client side:
Set-SmbClientConfiguration -EnableMultiChannel $false