I've recently noticed a very strange phenomenon on my Mac Pro 2009 (flashed to MacPro5,1) with a Xeon W5590 (Nehalem) using macOS 10.12.6: Slow downloads (5MB/s) over HTTPS in Safari and other Apple apps to some hosts (e.g. 102_hd_platforms_state_of_the_union.mp4) Activity Monitor is reporting network activity of 50MB/s, as is the router. There is definitely 500MBit/s of traffic flowing to the computer (which is still too little, but way more than Safari claims to get). `Safari Networking` consumes ~100% CPU (= 1 thread) while downloading Happens in all user accounts of this machine (including fresh ones); happens on a fresh install of macOS 10.12.6 as well The problem persists in Safe Boot Plain HTTP is fine (100MB/s) Chrome is fine (100MB/s) Other computers in the same network are fine (100MB/s) When using Charles proxy, the download looks normal, but seems to be CPU-limited to 15MB/s Looking at the call trace with Instruments, I see a lot of CPU time spent in AES decryption. My theory is that Apple at some point updated their TLS libraries to support Intel's AES-NI instruction set, which the Nehalem architecture lacks. Additionally, there seems to be some error in the fallback that leads to the weird behaviour I'm seeing (90% of traffic "lost"). Does anyone with a Nehalem Mac Pro and a fast internet link see the same? Update: The 50MB/s number is accurate and no traffic is actually lost: When all data is transferred, Safari will finish the download with speeds > 500MB/s. The data is buffered in RAM (as can be seen by the growing memory consumption of the `Safari Networking` process).