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I've noticed recently that my 2019 MBP 16' (running latest version of Big Sur) becomes very slow and sluggish after being connected to an external monitor for a certain amount of time (the time varies, sometimes it's 1 hour, 2 hours, 30 mins, etc). For some reason CPU usage goes to 80-90% across all cores when this happens, although the temperature remains steady at ~60C (so I don't think it's CPU throttling or anything like that). Opening Activity Monitor shows that kernel_task consumes most of that CPU, which I understand it to be part of the OS, although I don't know why it suddenly spikes. I've monitored GPU temps when this happens, and they also seem to be somewhere in the 60s C most of the time. I don't think that's hot enough for throttling, although I don't know for sure.

The issue persists whether I use an USB-C to HDMI dock, or a USB-C to DP cable. Disconnecting the external monitor for a few minutes resolves the issue, but it inevitably happens again once I reconnect it.

I've tried a few things like resetting the SMC (at least I think I have; the OS gives you no confirmation), or connected the power cable and monitor on different sides, but to no avail.

Any solutions to this problem? I'm using a Dell monitor, in case that matters.

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    See apple.stackexchange.com/questions/363337/… and apple.stackexchange.com/q/209575/85275 Basically, if kernel_task is ramping, then it's trying hard to cool your Mac.
    – Tetsujin
    Jun 10, 2021 at 16:49
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    That's the first thing I thought, but as I said, the CPU or GPU temperatures don't really increase as far as I can tell. They retty much remain steady in the 60s at all times when connected. The comment about connecting the charger on the right side of the Mac is interesting though. I have it on the left, and the monitor on the right. I'll try switching them and see what happens.
    – johnymm
    Jun 10, 2021 at 16:55
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    Wish you luck. That's always the thing about it being able to sit at a constant 60… you can't tell if that's entirely because of kernel_task's activity or not. I've a totally different Mac, but my CPU & GPU like to be at 50, fans ramp like all heck if it goes over 60
    – Tetsujin
    Jun 10, 2021 at 17:04
  • I'm also wondering if it's this specific monitor. It's a new monitor and I haven't had this issue before with other monitors, but I also haven't really used my mac with an external monitor that much before. So it's hard to tell. Perhaps I'll experiment with another monitor and see if the issue persists.
    – johnymm
    Jun 10, 2021 at 17:07
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    I'm having the EXACTLY same issue but it's not always been like that. It's only started to happen recently. It even happens if I do pretty much nothing with my macbook. Even if it goes in screensaver mode, the screensaver animation starts to lag even if temperatures are around 58-62 degrees. If I disconnect and reconnect the cable, kernel_task disappears even though temperatures remain unchanged. I tried different cables and monitors but the result is the same.
    – Beter
    Jun 12, 2021 at 12:27

1 Answer 1

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What did work for me is to put the one side of the laptop for HDMI dock and the other side of laptop for charging, this prevented the processor from the kernel task that consume most of the CPU

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