Check this if your experience slow upload speed on Windows WiFi
Disclaimer: It is recommended to use an AI agentic tool (like Codex) to inspect your computer settings first, and proceed only after the AI agrees on the operation.
TL;DR:
If your device meets the following criteria:
- Wi-Fi speed tests (e.g., speedtest.org, iperf3) show normal TCP download speeds, but extremely slow upload speeds (5–10 Mbps; increasing the number of threads yields no improvement)—significantly slower than other mobile devices in the same location—while UDP speeds remain normal.
- Speeds are normal when using a wired (Ethernet) connection.
- There is a virtual network adapter named “Network Bridge” (or similar) listed in your network connections (
ncpa.cpl). - You have previously used Microsoft virtualization technologies such as Hyper-V or WSL.
Then
Check the properties of this “Network Bridge” adapter under “Properties” -> “Advanced” (or via PowerShell: Get-NetAdapterAdvancedProperty). Look for two settings named “Large Send Offload v2 (IPv4)” and “Large Send Offload v2 (IPv6).” If they are enabled, disable them. Re-run the speed test, and speeds should return to normal. You may also be able to simply delete the “Network Bridge” adapter entirely.
Why?
As is well known, TCP data streams are transmitted in individual packets. When an application needs to send a large chunk of data (e.g., 128 KB), the network stack must split the data into segments based on the MSS (Maximum Segment Size, typically 1460 bytes), calculate checksums, populate headers, and so on. This segmentation process can be performed either by software (the CPU) or by dedicated hardware. The latter is known as Large Send Offload (LSO). When LSO is enabled, the software does not perform the segmentation itself; instead, it writes the data to be transmitted into a 64 KB buffer in bulk, leaving the hardware to handle the segmentation.
Wired network adapters almost always implement LSO. However, wireless adapters generally do not implement LSO due to their fundamentally different underlying transmission protocols. Yet, the “Network Bridge” created by Microsoft to support virtualization and network bridging requirements has LSO enabled by default—even though its only physical member, the wireless adapter, lacks LSO capabilities. Consequently, user applications would write upload data into a massive buffer—just as they would with LSO enabled—but the bridge would then re-segment and repackage this data in software before sending it to the wireless network adapter. Unfortunately, this “overambitious” behavior by the bridge severely hampered the computer’s efficiency in processing TCP upload data, thereby slowing down upload speeds. UDP and TCP download speeds (including ACK return packets) remained unaffected, as they do not pass through the TCP upload segmentation path.
I simply deleted this bridge, and so far, I haven’t encountered any issues. I suspect the bridge was created when I previously enabled bridged networking mode in WSL2. Since that mode was deprecated in December 2024 (effectively replaced by “mirrored” mode), deleting the bridge caused no problems.
I can finally enjoy a proper Wi-Fi experience—no more hunting for Ethernet cables to plug in!
- Title: Check this if your experience slow upload speed on Windows WiFi
- Author: Stargazer ZJ
- Created at : 2026-07-29 11:35:55
- Updated at : 2026-07-30 02:38:03
- Link: https://ji-z.net/2026/07/28/Check-this-if-your-experience-slow-upload-speed-on-Windows-WiFi/
- License: This work is licensed under CC BY-NC-SA 4.0.