Mac Runs Very Slowly on Connecting to External Display

I recently purchased a Brand New monitor to use with my Macbook Pro 2015 Model. The LED looks awesome and it was fun to experience OSX Sierra on a 21.5-inch screen. But it became miserable once I used it for 5 minutes.

What Happened?

My Mac became terribly slow and in 2-3 more minute, it became unusable. This was a personal disaster. I don’t expect such things happening to a Mac while connecting one external display. Macs are efficient enough to run more than 2 Displays at one time. Especially my model the one with 16 GB Ram 2.4GHz Quad Core Processor.

Possible Problems

After reading on Apple forums I discovered that my CPU had started running very fast and at high percentages.

Screen Shot 2017-06-30 at 10.55.53 AM

Ideal System/User Values

My System value was up to 65%, and only 20% of the CPU was idle. Now I assumed one of the following things is wrong:

  • Mac OS Sierra (because last time I connected an external display was with El Capitan and everything was fine)
  • Poor HDMI Cable (but my Dish + TV were working fine with the same HDMI Cable)
  • Problem with the Monitor itself
  • Problem with one of the ports

How Did I solve it?

  • New HDMI Cable.
    • Yes, that’s it. Even if your HDMI cable is working with other devices doesn’t necessarily mean that it would work with Macbook as well.

Other Solutions

  • Reset your PRAM or SMC (Less likely, but may work)
  • Clean your ports
  • Clear Mac Cache or Clean your Mac of as much junk as possible. (makes no sense, but works for some people)
  • In some cases, even a simple restart of Mac helped users solve this problem.
  • Try connecting your Mac with other displays.
  • Take Mac or Display to respective service centers.

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