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I was using my MacBook pro for years with two external full HD displays without a problem. Recently I switched to two 4K displays and since then fans in my MacBook spin all the time when when hooked up to external screens. I see that WindowServer spins at 20-30% CPU most of the time even if I do some basic activities.

Is this something expected or is it fixable and how?

I'm on High Sierra 10.13.6 with 16 GB ram, i7 and GeForce 750M with 2GB ram.

When I tried to watch some 4K youtube video I experienced stutter and overall poor playback quality. The same video in Full HD runs smooth.

[Edit]

I tried the same on Mid 2015 MacBook Pro equipped only with Intel Iris and it has no problem supporting 2 external 4k screens. Does one year in those models makes such a big difference? Or there is something wrong with mine?

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Assuming you went from 1080p to 2160p your total screen area has gone from 4 megapixels to 16 megapixels.
If you actually mean 'full 4k' 4096 × 2560, then your screen area is now in the order of 20 megapixels.
That's aside from the internal screen.

You're just expecting too much of it, to be able to push that kind of resolution at full speed.

Spec for the 2014 vs 2015 - source Everymac

  • 2014 - 2 external displays over Thunderbolt 2 @ 2560x1600
    or a single display up to 2560x1600 via Thunderbolt and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.

  • 2015 - 2 external displays over Thunderbolt 2 @ 3840x2160
    or a single display up to 3840x2160 via Thunderbolt 2 and a single 1080p display at up to 60 Hz, 3840x2160 at 30 Hz, or 4096x2160 at 24 Hz via HDMI.

So yes, it would appear the 2015 has significantly more grunt for video, even though it geekbenches about the same as the 2014 overall.

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    I tried the same config on mid 2015 MacBook equipped only with Intel Iris and it has no problem supporting 2 external 4k screens. Does one year in those models makes such a big difference? Aug 1, 2018 at 10:44
  • Answer updated.
    – Tetsujin
    Aug 1, 2018 at 10:55
  • Time for an upgrade then. Damn it. The thing works for years and I have no reason to upgrade. Apart from this :/ Too bad Apple just discontinued non-USB-C models. Aug 1, 2018 at 14:24
  • big screens take a lot of pushing, I'm afraid. My old workhorse I use as a media machine will be fine until I update my TV to a 4k or 8k, then it will have to go too :/
    – Tetsujin
    Aug 1, 2018 at 14:25
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I'm using 4k monitor (Dell U2720Q) with my MacBook Pro 15" mid 2014 on 60Hz connected through mini display port without any problems. USB hub works as well with USB-C -> USB-A cable (included).

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  • I have the same monitor and Mac, which cable are you using? Also does USB hub on monitor work with Mac? Mar 5, 2021 at 0:22

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