Hello Apple Support Team,
We are experiencing a performance issue with HTTP/3 in our iOS application during testing.
Problem Description: Network requests using HTTP/3 are significantly slower than expected. This issue occurs on both Wi-Fi and 4G networks, with both IPv4 and IPv6. The same setup worked correctly in an earlier experiment.
Key Observations:
- The slowdown disappears when the device uses: · A personal hotspot. · Network Link Conditioner (with no limitations applied). · Internet sharing from a MacBook via USB (where traffic was also inspected with Wireshark without issues).
- The problem is specific to HTTP/3 and does not occur with HTTP/2.
- The issue is reproducible on iOS 15, 18.7, and the latest iOS 26 beta.
- HTTP/3 is confirmed to be active (via assumeHttp3Capable and Alt-Svc header).
- Crucially, the same backend endpoint works with normal performance on Android devices and using
curlwith HTTP/3 support from the same network. - I've checked the CFNetwork logs in the Console but haven't found any suspicious errors or obvious clues that explain the slowdown.
- We are using a standard
URLSessionwith basic configuration. - Attempted to collect qlog diagnostics by setting the
QUIC_LOG_DIRECTORY=~/ tmpenvironment variable, but the logs were not generated.
Question: What could cause HTTP/3 performance to improve only when the device is connected through a hotspot, unrestricted Network Link Conditioner, or USB-tethered connection? The fact that Android and curl work correctly points to an issue specific to the iOS network stack. Are there known conditions or policies (e.g., related to network interface handling, QoS, or specific packet processing) that could lead to this behavior?
Additionally, why might the qlog environment variable fail to produce logs, and are there other ways to obtain detailed HTTP/3 diagnostic information from iOS?
Any guidance on further diagnostic steps or specific system logs to examine would be greatly appreciated.
Thank you for your assistance.