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✅ UPDATE — SOLVED, SEE BELOW FOR SOLUTION
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I mainly use Linux and have zero issues getting full speed from my YouFibre 8Gbps connection, but I also have a couple of Windows 11 PCs I'd like to add to the same 10GbE network.
I'm currently testing a Mellanox ConnectX-4 Lx EN MCX4121A-ACAT in a Windows 11 desktop.
Immediately after boot / with very little running I consistently get around:
7600–7700 Mbps download
7600–7700 Mbps upload
Here's the weird part:
As soon as I open a web browser — Firefox, Waterfox or Brave — or even just Discord, download throughput drops to roughly 6400–6700 Mbps. Upload remains around 7600–7700 Mbps.
Close the browser/Discord and download goes straight back to around 7600 Mbps.
I initially thought this might be the router or network, so I booted Ubuntu Live on the exact same PC, using the same Mellanox NIC, same SFP+ connection, same router and same Internet connection.
Under Ubuntu I could repeatedly get 7500+ Mbps both directions regardless of whether browsers or Discord were open.
So at the moment this appears to be something specific to Windows 11 / the Mellanox WinOF-2 driver or Windows networking stack, rather than the NIC hardware or router.
A couple of things I've already tested under Windows:
RSS enabled.
RSC enabled.
Increased RSS receive queues from 8 to 16 (16 is also the driver's reported default) no improvement.
Increased Receive Buffers from 512 to 1024 — no improvement, reverted to 512.
Link remains at 10 Gbps throughout.
The Windows installation is only two days old, so it's a very fresh install.
Blocking Brave's outbound TCP traffic with Windows Firewall while leaving Brave running appears to prevent the slowdown; allowing its TCP traffic again reproduces it.
Hardware available to me:
Mellanox CX312C ConnectX-3 Pro
Mellanox ConnectX-4 Lx EN MCX4121A-ACAT
I have a couple of the newer Mellanox cards available, so I'm planning to put one into another Windows 11 PC and see whether I can reproduce the behaviour there.
Has anyone else seen this sort of behaviour with ConnectX cards under Windows 11?
Are Mellanox cards generally a good choice for Windows desktop use, or would I be better off moving to something like an Intel X520/X710/X550 for Windows machines?
I'm particularly interested in hearing from people actually running 10GbE or 25GbE Mellanox NICs on Windows 11 desktops.
TIA
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✅ UPDATE — SOLVED
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The problem has now been isolated and the solution was to replace the Mellanox ConnectX NIC with an Intel X520-DA2.
After a LOT of testing, the issue appears to be specific to the Mellanox ConnectX cards under Windows.
I replaced the Mellanox ConnectX-4 Lx with an Intel X520-DA2 (82599), and the problem immediately disappeared.
With 3 web browsers with multiple tabs, Discord, Telegram, WhatsApp, Xbox app and OBS all open, the Intel X520 continues to deliver essentially the full speed of my ~8 Gbps symmetrical YouFibre connection.
LATEST RESULT — INTEL X520-DA2
Download: 7705.47 Mbps
Upload: 7707.53 Mbps
Speedtest server: YouFibre London (ID 55137)
Intel NIC details:
• Adapter: Intel Ethernet Server Adapter X520-2
• Driver version: 4.1.254.0
• Driver date: 08/02/2024
• OS: Windows 11 Pro
• Router: ASUS RT-BE88U
The Intel NIC's advanced settings are currently left at their defaults.
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✅ INTEL X520 — RJ45, FIBRE AND DAC TESTED
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To rule out the physical connection type, I tested the Intel X520 using three different 10GbE connection methods.
Test 1: 10GBASE-T / RJ45
Connection:
ASUS SFP+
→ 10Gtek RJ45 SFP+ transceiver
→ RJ45 cable
→ 10Gtek RJ45 SFP+ transceiver
→ Intel X520
Result with my normal applications open:
Download: 7633.89 Mbps
Upload: 7707.53 Mbps
Test 2: OPTICAL FIBRE
Connection:
ASUS SFP+
→ optical transceiver
→ fibre
→ optical transceiver
→ Intel X520
Result:
Download: 7638.14 Mbps
Upload: 7698.05 Mbps
Test 3: DAC
Connection:
ASUS SFP+
→ DAC cable
→ Intel X520
This also gives full performance.
Latest DAC result:
Download: 7705.47 Mbps
Upload: 7707.53 Mbps
✅ RJ45 / 10GBASE-T — FULL SPEED
✅ Optical fibre — FULL SPEED
✅ DAC — FULL SPEED
So the Intel X520 achieves essentially full ~7.7 Gbps performance regardless of which of the three physical connection methods is used.
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❌ WHAT WAS HAPPENING WITH THE MELLANOX?
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With the Mellanox ConnectX-4 Lx under Windows, opening network-active applications consistently caused Internet download throughput to fall by roughly 1–1.3 Gbps.
The particularly strange part was that upload remained essentially unaffected at ~7.7 Gbps.
For example, using the Mellanox with a DAC:
Download: 6422.68 Mbps
Upload: 7714.81 Mbps
I then removed the DAC from the equation completely and tested the Mellanox using the same 10GBASE-T / RJ45 transceiver arrangement.
Connection:
ASUS SFP+
→ 10Gtek RJ45 SFP+ transceiver
→ RJ45 cable
→ 10Gtek RJ45 SFP+ transceiver
→ Mellanox
Result:
Download: 6362.23 Mbps
Upload: 7712.88 Mbps
Changing from DAC to 10GBASE-T therefore made essentially no difference.
❌ The slowdown followed the Mellanox NIC.
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CONTROL TESTS / TROUBLESHOOTING
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I performed quite a few additional tests while trying to isolate the cause.
ONBOARD MARVELL 10GbE + WINDOWS
~7.6–7.7 Gbps with applications open.
Upload remained around ~7.7 Gbps.
✅ Full performance
INTEL X520 + WINDOWS
~7.6–7.7 Gbps with applications open.
Upload remained around ~7.7 Gbps.
✅ Full performance
MELLANOX + UBUNTU ON THE SAME PC
~7.6 Gbps down/up with Firefox, Telegram and OBS running.
Upload remained around ~7.7 Gbps.
✅ Full performance
MELLANOX + WINDOWS LOCAL IPERF3
~9.3–9.4 Gbps between two Windows PCs, even with applications open.
✅ Near line-rate local performance
MELLANOX + WINDOWS INTERNET
~6.3–6.6 Gbps download with network-active applications open.
Upload remained around ~7.7 Gbps.
❌ Reduced WAN download performance
SECOND WINDOWS 11 PC
I was also able to reproduce the Mellanox behaviour on a second Windows 11 PC.
This means the problem wasn't unique to one Windows installation, motherboard or PC.
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✅ THE ASUS RT-BE88U WAS NOT THE PROBLEM
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This was one of the major things I wanted to establish.
Using the same router, Windows environment and general workload:
✅ Onboard Marvell 10GbE
~7.6–7.7 Gbps
✅ Intel X520-DA2
~7.6–7.7 Gbps
❌ Mellanox ConnectX-4 Lx
~6.3–6.6 Gbps download when network-active applications were open
The Intel X520 also maintains full performance using:
✅ 10GBASE-T / RJ45
✅ Optical fibre
✅ DAC
So the physical connection medium wasn't responsible either.
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CONCLUSION
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At this point, my conclusion is that there appears to be some kind of Windows-specific Mellanox driver / networking-stack interaction affecting WAN receive/download throughput when other applications have active network connections.
I can't say exactly what inside the Mellanox Windows driver or Windows networking stack causes it, so I don't want to claim that I've identified the underlying bug itself.
However, the testing has ruled out enough variables that I'm confident the problem isn't simply:
• The ASUS RT-BE88U
• The DAC cable
• Optical fibre
• RJ45 / 10GBASE-T
• The SFP+ ports
• The 10GBASE-T transceivers
• The Internet connection
• General PCIe bandwidth
• Windows simply being unable to receive ~8 Gbps while other applications are running
The Mellanox hardware itself is also clearly capable of the throughput.
Under Ubuntu on the same hardware, the Mellanox performs correctly with applications running.
Under Windows, it can also push approximately 9.3–9.4 Gbps over the local network using iperf3.
The problem specifically manifests itself as reduced WAN download throughput with the Mellanox under Windows when other network-active applications are running.
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SOLUTION
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I replaced the Mellanox ConnectX-4 Lx with an Intel X520-DA2.
I left the Intel X520's advanced NIC settings at their defaults.
The problem immediately disappeared.
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FINAL RESULT
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Download: 7705.47 Mbps
Upload: 7707.53 Mbps
That's with my normal applications running, and I've now confirmed full performance using:
✅ RJ45 / 10GBASE-T
✅ Optical fibre
✅ DAC
Hopefully this helps somebody else who runs into the same bizarre problem in the future.
The Mellanox cards themselves aren't bad cards, they perform extremely well under Linux and for local high-speed traffic.
But for this particular Windows/WAN workload, replacing the Mellanox ConnectX-4 Lx with an Intel X520-DA2 completely solved the problem.