Windows 11 KB5089549 Slow Internet After Update Fix

What is the KB5089549 Slow Internet Issue?

After installing the Windows 11 cumulative update KB5089549 (released May 13, 2026), users on r/Windows11 and Microsoft Answers report that throughput drops 60-90% on both WiFi and wired Ethernet adapters. Speedtest.net results show typical 500 Mbps connections falling to 30-80 Mbps, with high ping spikes on real-time traffic (Discord voice, Zoom, gaming).

The root cause was traced to a regression in the new NDIS 6.85 driver model bundled with this update: TCP receive window auto-tuning gets stuck at a fixed 64 KB value, throttling any high-bandwidth stream. Microsoft acknowledged the issue on May 17, 2026 and a server-side Known Issue Rollback (KIR) is rolling out to consumer devices through May 22-24.

When does it occur?

• Immediately after the first reboot following KB5089549 install (OS Build 26100.4751)
• Most severe on Intel Wi-Fi 6E (AX211, AX210) and Killer Wi-Fi 7 BE1750 adapters
• Affects gigabit Ethernet on Realtek RTL8125 and Intel I225-V controllers
• More visible on connections above 200 Mbps (slow links may seem unaffected)
• Reproduces consistently in iperf3 tests against a LAN server
• Worsens when multiple TCP streams compete (Steam download + browser)

Common causes

• Stuck TCP receive auto-tuning level — the NDIS 6.85 regression
• Outdated WiFi driver shipped via Windows Update overriding the OEM driver
• Wi-Fi power management aggressively idling the radio
• IPv6 prefix advertisement timeout extended by the new networking stack
• Large Send Offload (LSO) disabled by the update on Realtek/Intel adapters
• Corrupted Winsock catalog entries left from the staged install
• QoS Packet Scheduler reservation reset to 80% on metered connections

Step-by-step fixes

1. Confirm the auto-tuning regression. In an elevated Command Prompt: netsh interface tcp show global. If "Receive Window Auto-Tuning Level" shows disabled or restricted, you have the bug.

1. Re-enable TCP auto-tuning with the value Microsoft recommends post-patch:

   netsh interface tcp set global autotuninglevel=normal
   netsh interface tcp set global rss=enabled
   netsh interface tcp set global chimney=disabled

Re-run a speed test — expect throughput to recover immediately.

1. Reset the Winsock catalog and TCP/IP stack if step 2 did not fully restore speed:

   netsh winsock reset
   netsh int ip reset
   ipconfig /flushdns

Reboot when prompted.

1. Roll back the Microsoft-pushed network driver to the OEM version. Open Device Manager -> Network adapters -> right-click your WiFi/Ethernet -> Properties -> Driver tab -> Roll Back Driver. If the rollback option is greyed out, download the latest driver directly from Intel, Realtek, or Killer (Rivet Networks) and install it manually.

1. Disable WiFi power saving that the update re-enabled. Device Manager -> Network adapters -> your WiFi -> Properties -> Power Management tab -> uncheck "Allow the computer to turn off this device to save power."

1. Re-enable Large Send Offload if your adapter supports it. Device Manager -> your adapter -> Properties -> Advanced tab -> set "Large Send Offload V2 (IPv4)" and "(IPv6)" to Enabled.

1. Force-install the Known Issue Rollback (KIR) on Pro/Enterprise via Group Policy. Download KB5089549-NetworkPerf-KIR.msi from the Microsoft Update Catalog, then gpedit.msc -> Computer Configuration -> Administrative Templates -> Microsoft KIR -> Enable. Reboot. Home users receive it automatically with the May 22-24 server-side push.

1. As a temporary workaround on Wi-Fi 7 hardware, force the adapter to 5 GHz only. Device Manager -> WiFi -> Advanced -> "Preferred Band" -> set to "5 GHz only" or "6 GHz only". The 2.4 GHz path is the worst-affected by the NDIS regression.

If it still doesn't work

If throughput is still capped after the KIR arrives, perform a full network stack reset via Settings -> Network & Internet -> Advanced network settings -> Network reset. This re-installs every adapter and clears all VPN/firewall hooks. For users running a VPN client (NordVPN, ExpressVPN, Cisco AnyConnect), update to the latest version — these clients ship their own TAP/TUN drivers that need recompiling against NDIS 6.85. If the issue persists only on a specific website, run tracert to identify if the slowdown is on your ISP path rather than Windows. As a last resort and only if you cannot wait for the KIR, uninstall the update from Settings -> Windows Update -> Update history -> Uninstall updates -> KB5089549 — note that this will be re-offered until the KIR rolls out to your device.