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 via HDMI 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, opening the case and cleaning dust out.


8 Answers 8


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?

You can test this by connecting a display of much lower resolution or decreasing the pixels being displayed in some cases. In other cases no amount of slight changes will reduce the CPU load you show. Since that fluctuates - what are the energy tab showing for the last hour(s) of usage?

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.

enter image description here


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!


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

  • This should be the accepted answer!
    – ExTexan
    Apr 1, 2022 at 11:32
  • 5
    Unfortunately not working on 12.4 (21F79)
    – cV2
    Jun 10, 2022 at 16:57
  • Tip: I had to hold the alt key while clicking "Scaled" for the resolutions to show up. I'm on Big Sur. Mar 8 at 15:52

Restarting SMC chipset helped fixing it.

  • Shut down Mac
  • Hold. CTRL+OPTION+SHIFT+POWER button for 10 seconds
  • Start mac
  • 2
    I got the same issue as OP. My MBP 2015 has an AMD Radeon R9. It had been working properly until I plugged an external monitor (Dell Ultrashap 24"). The OS became laggy. Unplug the monitor then it back to normal.
    – Andree
    Nov 4, 2021 at 12:50
  • 3
    @Andree is your experience also that one SMC reset is enough to know if that was causing inadequate cooling response instead of the Mac just being thermally throttled when the GPU is engaged.?
    – bmike
    Jul 19, 2022 at 12:16

After many tries, I found a solution that worked for me.

In my case, it was due to high temperatures on (I guess) my MacOS Pro 2019 ( and the HDMI adapter. After some tests, having a fan pointing to both the computer and adapter solved my problem!

  • 1
    What is a MacOS Pro 2019? Do you mean MacBook Pro?
    – Allan
    Mar 30 at 17:36
  • For me, just plugging in the HDMI directly into MacBook M1 Pro instead of through a dock station (which had keyboard and mouse plugged in as well) worked for me. Not sure if it's due to the dock station heating up or if it's slow in general.
    – Bugs Buggy
    Apr 3 at 16:43

The reason is heating your MacBook. If you can keep your MacBook cool using AC/Fan it will work better.


Lowering the refresh rate of the screen can help significantly as wellRefresh rate


After upgrading to Mac OS Ventura 13.3.1 version. My MacBook Pro slowness issue resolved. You can also upgrade.


I was having the same issue on my MBP 2017 running on MacOS Ventura 13.3.1 which had a 4k external monitor connected over HDMI.

Every time I played a youtube video(not even a 4K), the laptop was lagging and in the Activity manager I was seeing high CPU utilisation by kernel_task. There was no issue if I used the laptop without an external display.

After some experimentation, I figured out that the issue happened only when I connected the external display and power adapter on the same side of the laptop.

It fixed very easily after connected both on them on either side of the laptop, the display on the right and power on the left.

Before: enter image description here

After: enter image description here

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .