I have a Macbook Pro (Retina, Mid 2012) with the following configuration: 2.3 GHz Quad-Core Intel Core i7, 8 GB 1600 MHz DDR3, Intel HD Graphics 4000 1536 MB

I use Dell P2419H as my external monitor which I connect my macbook via HDMI cable. I have calibrated the external display profile using the settings > display. The issue is when my macbook is being charged and connected to the monitor it becomes excruciatingly slow to the point of not being of any use.

This does not happen when I am running my macbook on battery power and connect to the monitor.

In both the cases my macbook screen is open so the normal airflow is not hindered in any ways.

When I see my activity monitor it seems that on plugged in mode power the kernal task increases.

Here is a screenshot of activity monitor when laptop is running on batter and connected to monitor

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Here is how it looks when on plugged in power + external monitor

enter image description here

  • looks like the kernel_task temperature issue to me. If possible, try to plug in the power supply on a different side of the Macbook than your external monitor cable.
    – iOSapps.de
    Commented May 28, 2020 at 12:32
  • Both the monitor and macbook power supply are on a different outlets. Commented May 28, 2020 at 12:39
  • 1
    Also its a 2012 macbook so there is just one charging port, which is on the left Commented May 28, 2020 at 13:52
  • The 2012 MacBook uses the Magsafe charger. What happens if you just plug in the charger and not the monitor? However increased CPU is expected because you’ve got the integrated GPU - meaning it uses the CPU and system memory to drive that external monitor.
    – Allan
    Commented May 28, 2020 at 13:56
  • 1
    Also, it’s an 8 year old machine, it’s a good idea to open it up, clean out the dust and re-paste the CPU with new thermal compound. The stuff Apple uses isn’t the greatest to begin with. I’ve solved lots of cooling problems just doing this.
    – Allan
    Commented May 28, 2020 at 14:00

1 Answer 1


Restarting SMC chipset helped fixing it.

  • Shut down Mac
  • Hold. CTRL+OPTION+SHIFT+POWER button for 10 seconds
  • Start mac

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