Technical Tip: SMB speed on IPsec depending of the WAN quality

The SMB protocol is designed for local file sharing with low latency. Even a slight packet loss or delay in these exchanges can cause noticeable slowdowns. Therefore, it is highly sensitive to packet loss rate, out-of-order, latency, and other issues in network communication, meaning even a small issue can significantly impact its performance. 

Example:

PC -> 40F (Spoke) -> 1G (IPsec)-> (Hub) -> Server.

Without Delay and Packet Loss, SMB speed is around 70MB/s in PC, almost 500 Mbps in an IPsec tunnel in Spoke.

With 5ms Delay and without Packet Loss, SMB speed is around 56MB/s in PC, almost 400 Mbps in IPsec tunnel in Spoke.

With 5ms Delay and 6% Packet Loss, SMB speed is around 9MB/s in PC, almost 52 Mbps in IPsec tunnel in the Spoke.

Compare with Reorder Corrupt Duplicates, such as out-of-order, corrupt, affecting speed much faster than others:

No Delay, no packet loss, 6% Corrupt, SMB speed is around 27.9MB/s in PC, almost 214 Mbps in IPSec tunnel in Spoke.

Different delays cause different max speeds:

Conclusion:

1. SMB is not suitable for file transfer over a WAN link; it is for local file sharing.
2. SMB speed depends on the WAN link quality.

Additional suggestion:
SMB is a protocol the throughput that can be affected by latency on the path, consider the following:
Based the following Windows docs, SMB has some register options to control the throughput:
Slow SMB files transfer speed.

Disable the 'bandwidth throttling', and test again.

• Performance tuning for SMB file servers.
• Set-SmbBandwidthLimit.
  
  

Note: It will also happen for SSL VPN: 
SMB (Windows file sharing) performance over SSL VPN is often slow by design, and this behavior is expected. It is not caused by a fault in the FortiGate, but by how the SMB protocol behaves when used over encrypted WAN tunnels.