macOS Tahoe 26.x / iOS 26.x - Extremely slow upload speed (0.6 Mbps) - SOLUTION FOUND

Hi everyone,

after updating to macOS Tahoe 26 (and also iOS 26), I noticed a serious issue with network performance on my devices. I wanted to share what I’ve observed and a possible workaround, in case it helps others experiencing something similar.

Issue observed:

• Upload speed dropped from ~21 Mbps to ~0.6 Mbps

• Ping increased from ~15 ms to ~140 ms

• Download speed remained normal (~87 Mbps)

• The issue occurs on both Ethernet and Wi-Fi

• The same network works correctly on macOS Sequoia (15.x) and iOS 18.2

My Environment:

• MacBook Pro 14” (Nov 2023), M3 Max, 36 GB RAM

• Router: TP-Link Deco BE25 (Wi-Fi 7)

• ISP: EOLO (Italy)

Troubleshooting:

After various tests using iperf3 and other network tools, I noticed the following:

• ICMP ping behaves normally (~14 ms)

• TCP upload traffic stalls shortly after starting

• The behavior seems related to ECN (Explicit Congestion Notification)

Possible workaround on macOS:

Disabling ECN appears to restore normal performance on macOS. This can be tested by running the following command in Terminal:

sudo sysctl -w net.inet.tcp.ecn_initiate_out=0

To make the change persistent after reboot:

echo "net.inet.tcp.ecn_initiate_out=0" | sudo tee -a /etc/sysctl.conf

Results after applying the workaround:

• Upload: ~21 Mbps

• Ping: ~16 ms

• Download: unchanged (~87 Mbps)

iOS / iPadOS

On iOS and iPadOS, I haven’t found a way to disable ECN. As a temporary workaround, using a VPN (for example, Cloudflare WARP) seems to improve upload performance.

Additional notes:

ECN (Explicit Congestion Notification) is a TCP feature intended to improve congestion handling, but some network devices may not fully support it. It’s possible that something has changed in how ECN is handled in macOS/iOS 26 compared to earlier versions.

I'm not sure if this is a known issue or if it affects specific network setups, but I wanted to share my experience.

Is anyone else seeing similar behavior after the update?

Andrea