Slow Internet when using the SonicWALL Global VPN client

If you’ve ever connected to the SonicWall Global VPN Client on Windows 10 and immediately watched your internet speed fall off a cliff, you’re not imagining things—and you’re definitely not alone.

Across multiple environments I’ve supported—everything from small professional offices to mid-sized enterprise networks—I’ve repeatedly seen the same pattern:

• Users achieve 50–90 Mbps internet speeds off VPN
• They connect to SonicWall Global VPN
• Speeds collapse to sub-1 Mbps
• Web browsing becomes painful
• Teams and VoIP calls stutter
• File transfers crawl

Naturally, the VPN or firewall gets blamed. And honestly, that’s a reasonable assumption—until you realise:

• The VPN connects cleanly
• There are no firewall errors
• Logs show no packet drops
• Wired connections often work fine

After chasing encryption settings, client versions, MTU values, and firewall rules more times than I care to admit, the real culprit turned out to be something far less obvious:

👉 A Windows 10 Wi-Fi networking feature that does not play nicely with the SonicWall Global VPN Client.

This article explains why the issue occurs, how to fix it properly, and what to check before redesigning your VPN or replacing hardware unnecessarily.

The Symptoms: How This Issue Typically Presents

When this problem appears, the behaviour is usually very consistent:

• Internet speeds drop dramatically only when VPN is connected
• Speed tests show <1 Mbps download
• Internal resources may also feel sluggish
• Disconnecting the VPN immediately restores normal speeds
• The issue occurs far more often on Wi-Fi than Ethernet

In my experience, it shows up most often with:

• SonicWall Global VPN Client 4.10.x
• Windows 10 (Creators Update and later)
• Modern wireless adapters with advanced offload features enabled

That last point is the key—and it’s where most troubleshooting guides stop short.

Why This Happens (The Bit Most Articles Skip)

The Real Root Cause: Receive Segment Coalescing (RSC)

The underlying issue is a Windows networking feature called Receive Segment Coalescing (RSC).

What Is Receive Segment Coalescing?

RSC is designed to improve network performance by:

• Combining multiple TCP segments into larger packets
• Reducing CPU interrupts
• Improving throughput on high-speed networks

On paper, it’s a good optimisation—and for most workloads, it works well.

Why RSC Breaks SonicWall Global VPN Performance

The problem appears when kernel-level VPN drivers, like the SonicWall Global VPN Client, interact with RSC-enabled Wi-Fi adapters.

Here’s what happens under the hood:

• Windows attempts to coalesce incoming packets
• The VPN client expects packets in a specific format
• Packet handling becomes inefficient
• TCP retransmissions increase
• Throughput collapses

This interaction hits Wi-Fi adapters hardest, which explains why:

• Ethernet connections often work perfectly
• Only certain laptops are affected
• Changing VPN encryption settings makes no difference

This is not a firewall misconfiguration.
This is not a bandwidth issue.
This is a Windows networking stack compatibility problem.

The Proven Fix: Disable Receive Segment Coalescing (RSC)

Microsoft is aware of this behaviour and provides both an automated fix and manual configuration options.

Disabling RSC on the affected Wi-Fi adapter resolves the issue almost immediately in the vast majority of cases I’ve seen.

Option 1: Microsoft’s Recommended Automated Fix (Fastest)

If you’re supporting end users or want the quickest resolution, Microsoft provides an official diagnostic tool.

Steps

1. Download the Microsoft diagnostic tool:
   👉 https://aka.ms/diag_cssemerg11005
2. Run the downloaded .diagcab file
3. Allow the tool to detect and apply the fix
4. Reboot the machine (recommended)
5. Reconnect the VPN and re-test performance

In most real-world cases, internet speeds return to normal instantly.

Option 2: Disable RSC Manually (Preferred for IT & Enterprise Environments)

In locked-down environments where diagnostic tools are blocked, manual configuration is often the better approach.

Step 1: Open an Elevated Command Prompt

Start → Windows System → Command Prompt
Right-click → Run as administrator

Step 2: Identify the Network Adapter

Get-NetAdapter

Take note of the adapter name (commonly Wi-Fi).

Step 3: Check Current RSC Status

Get-NetAdapterRsc

If RSC is enabled, you’ll see:

• IPv4Enabled : True
• IPv6Enabled : True

Step 4: Disable RSC on the Adapter

Disable-NetAdapterRsc -Name "Wi-Fi"

(Replace "Wi-Fi" with your actual adapter name.)

Step 5: Verify RSC Is Disabled

Get-NetAdapterRsc

You should now see:

• IPv4Enabled : False
• IPv6Enabled : False

Step 6: Re-Test VPN Performance

Reconnect to the SonicWall VPN and run a speed test. In most cases, throughput returns to expected levels immediately.

Why This Fix Works So Reliably

Disabling RSC forces Windows to:

• Process packets individually
• Avoid aggressive TCP aggregation
• Deliver traffic to the VPN driver predictably

Yes, this slightly increases CPU usage—but on modern systems, the impact is negligible. In dozens of deployments, I’ve yet to see any negative side effects from disabling RSC on Wi-Fi adapters used with VPN clients.

Additional Checks Worth Doing (But Rarely the Root Cause)

Before escalating the issue or redesigning your VPN architecture, also verify:

• Wireless drivers are up to date
• No third-party packet filtering or endpoint agents are interfering
• MTU values are left at default (unless explicitly required)
• The issue does not occur over Ethernet

If Ethernet works flawlessly and Wi-Fi does not, that’s a strong indicator you’re dealing with an RSC-related problem.

Final Thoughts: One to Keep in Your Troubleshooting Toolkit

This is one of those issues that can consume hours of troubleshooting if you don’t know where to look.

The key takeaways:

• SonicWall Global VPN Client itself isn’t inherently slow
• Windows 10 networking optimisations can break VPN performance
• Wi-Fi adapters are disproportionately affected
• Disabling Receive Segment Coalescing is a proven, low-risk fix

If you support Windows VPN users regularly—especially in mixed Wi-Fi environments—this is absolutely one to keep in your back pocket.

Daniel Lutton

From my early days on the helpdesk through roles as a service desk manager, systems administrator, and network engineer, I’ve spent more than 25 years in the IT world. As I transition into cyber security, my goal is to make tech a little less confusing by sharing what I’ve learned and helping others wherever I can.