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I'm currently using a Mid-2015 MacBook pro and running Catalina's latest build on it. Although for quite a long time anytime I connect the MacBook to an external display it seriously slows down. when I check the usage via activity monitor I'll notice a huge spike in activities even if I'm not doing anything on the system

Connected to display Spike drops immediately I disconnect the display

I've tried so many things like mission control, Pram reset e.t.c

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    Kernel_task ramping up like that is trying to cool your machine down by making the fans spin up. I'd suspect perhaps your airflow isn't clean so when the GPU has to work twice as hard supporting the external screen, the machine can no longer keep cool. When was it last stripped & cleaned? – Tetsujin Apr 11 '20 at 12:09
  • Been a while @Tetsujin – TOLULOPE ADETULA Apr 11 '20 at 12:15
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    That would be the first task then ;-) – Tetsujin Apr 11 '20 at 12:15
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    It's become my new "lockdown sport" - clean one of the computers every day; takes about 2 hours to really do it properly on the big old Mac Pros. But you know what they say - 'An Apple a day keeps the doctor away' ;)) – Tetsujin Apr 11 '20 at 12:19
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    Is this the 2015 MacBook Pro with the integrated graphics or the one with the Radeon chip? If it’s the IG, it shares graphics processing with the CPU so an extra monitor is extra load. – Allan Apr 11 '20 at 12:55
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Kernel task exists to slow things down when the system gets hot. Have you ruled out a temperature rise when running more pixels adds GPU and CPU load? Especially if you run closed clamshell mode.

I would expect your Catalina activity monitor to show a GPU tab but maybe that’s only on some hardware. I also would be sure you’re running the latest EFI and firmware and reinstall the OS from recovery on top of your system. Check system information and build which are circled in red.

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Restarting SMC chipset helped fixing it.

  • Shut down Mac
  • Hold. CTRL+OPTION+SHIFT+POWER button for 10 seconds
  • Start mac
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enter image description hereExperiencing lag, slowing down mac when connecting an external monitor. We have had this experience with several Macs, even a MacPro 2020 with everything maxed out.

  • Do the following to resolve this:

  • Go to display settings

  • Switch "Default for Display" on your external monitor display settings

  • Hold OPTION and click Scaled

  • Check "show Low-Resolution modes"

  • Select desired resolution with (low resolution) attached to it

  • Mac Speed Normal!

Enjoy!

Youtube Video: https://youtu.be/RAk060t_8S4

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