Oh yes indeed it seems I got confused. It's usually my colleagues that submit bug reports... It's now done, here is the bug report number: FB20871628 Thank you for the precision! Pierre

Thank you Quinn. I originally did file a bug report (Case-ID: 16602242), but was told to post on the forum instead. So I pinged back on the ticket to let them know. One more thing that I've just learned from some of our customers: apparently there is no issue with macOS 15.3 but there is with macOS 15.6.1 So it seems the issue started in between these versions, I'll try to find a way to install 15.4 and 15.5 and confirm in which of these versions it started. Thank you again for your replies. Pierre

Hi Quinn, Thank you for your quick answer. You are correct, the issue is that the C:D ratio is wildly different than the A:B ratio. To be more specific, below are the results we can see to download a 20MB file, depending on your cases above (A, B, C or D). 1. Server on macOS 26.1, or macOS 15.6.1 With TSO enabled: A: Download time: 0.208543s, Bytes: 20971520, Speed: 100562090 bytes/sec B: Download time: 0.638284s, Bytes: 20971520, Speed: 32856095 bytes/sec C: Download time: 23.000043s, Bytes: 20971520, Speed: 911803 bytes/sec D: Download time: 3.259781s, Bytes: 20971520, Speed: 6433413 bytes/sec As you can see, with ethernet, a server in GCD takes less than 1 second to download a 20MB file, while with Network, it takes more than 20 seconds. In Wifi, it's slightly better, but still slower than GCD. With TSO disabled: If on the server side, we disable TSO using sudo sysctl -w net.inet.tcp.tso=0 A: Download time: 0.208460s, Bytes: 20971520, Speed: 100602129 bytes/sec B: Download time: 0.672691s, Bytes: 20971520, Speed: 31175562 bytes/sec C: Download time: 1.865778s, Bytes: 20971520, Speed: 11240093 bytes/sec D: Download time: 2.332780s, Bytes: 20971520, Speed: 8989926 bytes/sec It is much better for case C. 2. Server on macOS 14.7 With TSO enabled: A: Download time: 0.232860s, Bytes: 20971520, Speed: 90060637 bytes/sec B: Download time: 0.680401s, Bytes: 20971520, Speed: 30822294 bytes/sec C: Download time: 6.255250s, Bytes: 20971520, Speed: 3352626 bytes/sec D: Download time: 7.912810s, Bytes: 20971520, Speed: 2650325 bytes/sec A server using Network is still slowed than GCD, but it is much better than on macOS 26 and 15. With TSO disabled: A: Download time: 0.236461s, Bytes: 20971520, Speed: 88689128 bytes/sec B: Download time: 0.707016s, Bytes: 20971520, Speed: 29662016 bytes/sec C: Download time: 6.275888s, Bytes: 20971520, Speed: 3341602 bytes/sec D: Download time: 6.758729s, Bytes: 20971520, Speed: 3102879 bytes/sec Difference is marginal, so disabling TSO has no impact on macOS 14.7. I hope it clears things up. Let me know if you need more info. Pierre

Hi Quinn, Thank you for getting back to me. Yes I do mean macOS 26 or macOS 15. Earlier macOS versions do not show the issue. Note that we only tested with macOS 15.6.1, macOS 26.0.1 and macOS 26.1 beta 3 and 4. In the sample code and instructions that I provided, we can reproduce it with curl, but we have seen the issue first in our code where we use URLSession. Let me know if you need additional information. Pierre

Here's a link to download a sample code that will run a server with the Network API and with the GCD API. Instructions are included, in the "Instructions.txt" file, as you may have guessed. ;-) https://softron-downloads.s3.amazonaws.com/bvcwb546r7ret8hyj9tpm413a57zagf6qdf6h8/DualHTTPServer.zip Thank you! Pierre