Whenever I boot the machine, or wake it from sleep, with an external monitor connected, the laptop screen flicker and dim/undim for at least a whole minute (while the external display remains black) before finally turning on. While this is going on the machine is also very slow, and responds poorly to keyboard/trackpad input. This does not happen if I do not have an external monitor connected.


  • High Sierra 10.13.2 (17C88)
  • Macbook Pro (15", 2017)
  • External monitor is connected through an USB-C Digital AV Multiport Adapter (HDMI)
  • The machine is connected to a power cable

What I've tried

I've tried resetting NVRAM and SMC to no avail.

  • 1
    We're missing a lot of details here. How is this monitor connected? Which MacBook? Which build of High Sierra? Are you connected to power or on battery? Remember, we're not there to look over your shoulder. – Allan Dec 29 '17 at 14:01
  • @Allan See updated question. – o01 Dec 29 '17 at 14:39
  • My first thoughts are the "official" Apple AV adapter is flaky at best and that it would be beneficial to go DisplayPort to DisplayPort rather than HDMI. But if you must go HDMI, use an active adapter. What monitor are you using and does it support DisplayPort? – Allan Dec 29 '17 at 15:07

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