Customers that are using the SMB/SAMBA protocols to perform file transfers might notice less Bandwidth being utilized than what they would originally expect. Observation might also be that only one of the available links is being used partly, even though more than 1 link is available to carry the SMB/SAMBA traffic
SMB/SAMBA protocol is very sensitive to packet loss and implements a very aggressive flow control mechanism. Even if there is packet loss of less than 1% on a link used for the transfer, the impact to the bandwidth utilized being used for a file transfer would be major. The SD-WAN remediation mechanisms will improve the behavior but still the user will not see their transfer utilizing the full of the available Bandwidth of their links.
Few things might help to improve customer scenario in high loss and jitter links.
The highest protocol version currently available is SMB 3.1.1.
SMB 3.0 was introduced in Windows Server 2012 and further enhanced in Windows Server 2012 R2 (SMB 3.02) and Windows Server 2016 (SMB 3.1.1). This version introduced technologies such as SMB Multichannel, that may significantly improve performance and availability of the file server.
SMB Multichannel
SMB Multichannel allows file servers to use multiple network connections simultaneously and provides increased throughput.
Performance tuning for SMB file servers
https://learn.microsoft.com/en-us/windows-server/administration/performance-tuning/role/file-server/smb-file-server
Slow SMB files transfer speed
https://learn.microsoft.com/en-us/troubleshoot/windows-server/networking/slow-smb-file-transfer
Please ask the customer to retest with SMB version 3.1.1.