Until three months ago, I could use two external monitors with no problem. For 3 months I just used one, but now, trying two, I find that my MacBook Pro (15-inch, 2017, OS 10.15.5) gets slow--for example, my mouse pointer lags. In fact, even with one external monitor there is some lag.

How can I prevent this?

Full configuration is shown below.


1 Answer 1


Is the laptop getting enough power? This model of MacBook Pro has two graphics chips and will shutdown one of them if it sees a need to conserve power. That could explain the video lag. Are you using the original power supply? Is there some device plugged in that could be drawing significant power from the laptop?

  • Thank you. I am using an original power supply. The only devices are the monitors, and a dongle for a Microsoft wireless mouse and keyboard. Turning off the mouse and using an Apple bluetooth mouse seems to help.
    – Joshua Fox
    Jun 21, 2020 at 10:33
  • How can I tell if one graphics chip is shut down?
    – Joshua Fox
    Jun 21, 2020 at 10:34
  • I found an Apple support page that should help: support.apple.com/en-us/HT202053 It's saying to check the Activity Monitor and the Energy Saver settings in System Preferences.
    – MacGuffin
    Jun 21, 2020 at 10:47
  • Thank you! That Energy Saver setting looks useful. The slowness is no longer happening, but if it does, I will set that. There is no need to save energy when on AC power.
    – Joshua Fox
    Jun 21, 2020 at 10:53

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