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I have a Macbook Pro 16" and recently purchased a 24" (HD) Dell monitor via HDMI (via a USB-C to HDMI converter)

After about 20m of usage, my mac comes to a crawl and really slows down. It becomes impossible to work on the machine.

Plugging out the monitor fixes the issue in no time and I can see the kernel_task process (in Activity monitor) hit 900% to 1500% of CPU.

Specs

I am running macOS Big Sur
It is a 2019 model with Intel Core i9, 16 Gigs of RAM and 
the AMD Radeon Pro 5500M 4 GB graphics processor
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    Can't add an answer, so I will just comment, I tried every solution on the web, switching sides of power and monitors, using a dock, shutting down true tone, hardware acceleration, i basically wasn't able to use my 2019 macbook pro when connected to an external monitor, the only 2 things that worked for me were: 1) using macbook in clamshell mode, but you loose cam, mic, fingerprint, trackpad etc..., 2) and I CAN'T STRESS THIS ENOUGH, CLEAN YOUR FANS!! (by a pro or if you know what you're doing) then SMC reset, see my before and after pics here, i.imgur.com/ChgoGlj.jpg
    – bentael
    Mar 16 at 21:10

1 Answer 1

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After spending weeks of trying out various things: different HDMI cables, different USB-C to HDMI dongles, cleaning the fans and even leaving the air conditioner on to cool the room, the issue turned out to be:

Plugging the power cable on the left-hand side of the Macbook.

Solution:

Plugging the power cable to the right-hand side of the MacBook and the monitors to the left-hand side, solved the issue for me immediately.

Came across this Forbes article: https://www.forbes.com/sites/barrycollins/2020/04/24/why-you-shouldnt-charge-your-macbook-pro-from-the-left-hand-side/?sh=6209dcda78ff

Which I can now say, works. So if you are experiencing something similar and if your power cable is on the left hand side, moving it to the right hand side may fix it for you.

The other thing I would try (which I did) was to clean out your fans if this still does not work for you.

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    This does help, but equally essential is to use the MacBook Pro 16 inch in clamshell mode with an external monitor. GPU power consumption drops from approx. 18.8w to 5.8w. Even then fans run a lot when doing anything which taxes the GPU. It's an ill-conceived unit for use with an external monitor. Sep 22, 2021 at 0:19
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    @Foliovision I fully agree with your last statement and find it astonishing! I have been using external monitors on mac laptops for 20 years and never had such problem before. It is quite ridiculous that 2022 computers cannot do what 1994 computers could.
    – chris
    Jan 29 at 10:22
  • Chris, if you can get rid of the 2019 MacBook Pro 16 inch in favour of one of the M1 Pro or M1 Max, they handle much, much better. Almost always silent, easily manage two external monitors plus built-in monitor with no power issues. I've eventually chosen an M1 Max as it leaves the option for three or four external monitors and am using it as primary and only computer with 32GB of RAM. Finally a true desktop replacement. Feb 4 at 23:01
  • Its related to some macOS update. My existing monitors which used to work well with my Macbook Pro started slowing down the machine recently discussions.apple.com/thread/253860233 Apr 29 at 2:49

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