2026 Fix for Slow SMB Performance on Cisco Secure Cloud VPN

026 Fix for Slow SMB Performance on Cisco Secure Cloud VPN, Tom here from Orange Systems. A question that comes up a lot is: why are my file transfers so slow over a VPN?

The short answer is that SMB protocol was not designed for high latency connections. Server Message Block has gone through several versions and iterations, and each one has made some improvements, but those improvements still do not escape the fact that it is a protocol that has an acknowledgment back and forth as the data transfers. Therefore, any high latency causes a big slowdown.

Demonstrating the Problem: Low Latency vs High Latency

To demonstrate this, we are going to do two file transfer tests. The first test will be with low latency, and the second test will have just a little bit of latency added, similar to what you may see over a standard VPN connection.

For this demonstration, Linux is being used, but this is all about the protocol and not about which operating system. Linux simply makes it easier to display the transfer statistics.

We are going to be moving a file to an SMB 3.0 mount at 192.168.1.8, mounted under /mount/demo.

Test 1: Low Latency Transfer

The first thing to do is ping the target. The results show about a quarter millisecond average. It sometimes pings a little bit higher because of routing through a firewall, and routing SMB through a firewall is not the best idea in terms of transfer speeds. The same rule applies: once you add latency, you add problems.

On this low latency connection, a file of about 2.7 GB was copied using rsync. The transfer ran at about 113 MB per second and took just under 21 to 22 seconds to complete. The average came out to about 108 megabytes per second with the file completing in 21 seconds.

Test 2: Adding 30 Milliseconds of Latency

Now, a limiter was applied to add 30 milliseconds of latency. This is still a reasonably fast connection, but it is the kind of latency you may see over a VPN.

With the ping now sitting at about 29 to 30 milliseconds, the same file transfer was run again. The result was a substantial slowdown. What previously took 20 seconds now takes close to a minute to transfer, and that is only with that small amount of added latency. There is no packet loss and the bandwidth remains the same. The only difference is the delay added to the packets, and that delay is where the problem is.

To put it in perspective, the transfer speed dropped from 108 MB per second down to 31 MB per second just from adding 30 milliseconds of latency.

Why VPNs Make This Worse

When you use any type of VPN, there are always going to be extra hops between where you are and the remote systems. This applies no matter what you are using, whether it is Tailscale, WireGuard, ZeroTier, or any other VPN protocol. They all have slight optimizations that may work slightly faster, but the fundamental problem remains the same.

The easiest way to check whether or not you are going to have a slow transfer problem is to simply ping from the device you are connecting from to the server you are connecting to and check the latency. Once you get anything over a millisecond, as shown in this demonstration with only 30 milliseconds added, you can notice that the SMB protocol slows down substantially.

You may find some articles from Microsoft about them revamping SMB to work over WAN connections. It used to be even worse, so it has improved, but it still does not handle high latency well.

FAQ

Why are my file transfers slow over a VPN even though my VPN speed test is fast?

A speed test measures raw bandwidth, but file transfers using the SMB protocol require constant back and forth acknowledgments between the client and server. Any added latency, even just a few milliseconds, dramatically slows down this process regardless of how much bandwidth you have.

What is SMB and why does latency affect it so much?

SMB stands for Server Message Block. It is a protocol used for file sharing over a network. It was not designed for high latency connections because it requires acknowledgment messages back and forth during every data transfer. High latency means each acknowledgment takes longer, which slows everything down.

How can I check if latency is causing my slow file transfers?

Simply ping the server you are connecting to from the device you are using. If the latency is consistently over a millisecond, especially over a VPN connection, that is likely the cause of your slow transfers.

Does it matter which VPN protocol I use?

All VPN protocols, including Tailscale, WireGuard, and ZeroTier, introduce some extra hops and latency. They each have slight optimizations, but none of them can fully solve the underlying problem of SMB performing poorly over high latency connections.

Has Microsoft improved SMB for use over VPNs or WAN connections?

Yes, Microsoft has revamped SMB in newer versions to better handle WAN connections and it has improved over time. However, it still does not perform well under high latency conditions compared to protocols designed for such environments.

Conclusion

Hopefully this saves you some trouble before you spend a lot of time trying to tune settings, only to find that the fundamental problem is that the SMB protocol simply does not work well over high latency connections. This question comes up all the time, which is why this demonstration was put together to help people understand why a VPN speed test can come back fast, but a simple file transfer still feels painfully slow.