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I am using a Mac mini for playing media from a local network media server connected by cable (Ethernet). This Mac stays on 24/7 and is prohibited from accessing the Internet (router firewall).

Recently I noted that 4K movie playback had become jerky. Upon investigation I found out that the top speed at which the Mac could retrieve data from the media server was only about 11 MB/s (whereas other computers on the network were getting x10 faster speeds).

I simply rebooted the Mac and — bingo — the speed skyrocketed to 110 MB/s, and 4K movies started playing fine again.

What was that? How can I investigate what was throttling the speed?

If it matters, this is 2020 model (M1), Big Sur 11.5.1.

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    there is so little info, it is impossible to say what caused this. Further, if this has happened once since you set it up I would not worry at all. Investigating this without it being repeatedable is completely useless
    – X_841
    Jan 26, 2022 at 12:32
  • @X_841 The point of the question is to have some idea what to look for when it happens again. I don't want to simply reboot it next time.
    – Greendrake
    Jan 26, 2022 at 12:34
  • Similar issue. I am on a gigabit internet service. My iMac seems to be throttling only the download speed (~80Mbps). The Upload speed is still in the 900 Mbps range. What could be causing this disparity? It happens on both Ethernet and WiFi connections.
    – SkyKetchup
    Nov 10, 2023 at 23:55

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There could be many reasons for the reduced speed - it is impossible to know with the information you have gathered so far. It could be a problem with your media server, the local network (switches, etc.), the network cable directly connected to the Mac, hardware and/or settings on the Mac itself.

However, it does sound likely that your problem was due to link speed negotiation. On a standard gigabit ethernet system, the switch and Mac would normally negotiate a 1000 Mbps speed, which corresponds with your 110 MB/s figure. However, if the cable is not of the right quality, it is broken, or there's a sufficient source of electromagnetic noise nearby, then the two ends could be forced to negotiate a 100 Mbps speed instead. This corresponds with you observing a 10x reduction in speed.

If the problem occurs again, you can check by opening System Preferences > Network > Ethernet > Advanced > Hardware and look at the given speed.

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  • Yup. Agree. I have one flaky ethernet cable [long-distance, 'unfixable'] in my home setup. This periodically affects the one Mac linked through it. I found it by trial & error, going right through every connection unplugging, plugging, until I found the culprit. Now when it happens I just go straight to that plug; pull, wait 5s, push, & away we go again for another month or three. [Yes, I've tried everything to fix it, but it resists my attempts. I settled on this as being simpler than ripping the entire run out & starting over] ;)
    – Tetsujin
    Jan 27, 2022 at 10:00

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