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I have a new MacBook Pro 13" (mid-2018 Touch Bar). I'm connecting an LG 27UD88 external display directly via the supplied USB-C cable, which is also used to provide power to the laptop plus allows the display to act as a hub for USB-A devices.

The screen achieves 4K @ 60Hz via the USB-C ports on the left-hand side of the laptop. However, when I connect it via either of the ports on the right-hand side, it maxes out at 30Hz.

I have tried resetting the PRAM/NVRAM and SMC, and tried the cable in different orientations.

When I option-click Scaling in Display Prefs, the refresh rate is limited to 30Hz with the right-hand ports.

I have found other questions which describe 60Hz/4K issues with dongles (1, 2, 3), and mention of this left-hand/right-hand disparity on Reddit, but there is nothing concrete about whether this is a universal issue.

My question is: is this a fault with this particular unit, or is this a known limitation of all 2018 13" MacBook Pros?

Update: I have found that it is able to drive another 4K display at 60Hz from the right-hand ports, via a USB-C to DisplayPort cable. I'm wondering if the problem above is associated with the fact that the laptop is also drawing power and USB hub functionality from the LG display.

Daniel Jones
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    Just chiming in to say that I've had the same issue on at least two different 2018 MBPs. On one of them, I would sometimes get flaky 4k60 and sometimes 4k30; on the replacement, I now only get 4k30 on the right side. It feels like a hardware bug to me.

    Meanwhile, my colleague can do 4k60 on either side with the exact same machine and monitor...

     – lxgr Jan 09 '20 at 13:42

2 Answers2

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In 2016 and 2017, the left-hand Thunderbolt ports were higher bandwidth than the right-hand ones, and this would be expected behaviour. For 2018 models, all 4 ports should have the same performance.

So is it possible that you have a 2017 model without knowing it? If not, then I think this is unexpected behaviour.

Frank
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    Thanks for your answer. Unfortunately, About This Mac and System Report confirm that I have a 2018 model: "MacBook Pro (13-inch, 2018, Four Thunderbolt 3 Ports)". – Daniel Jones Apr 02 '19 at 18:56
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    The monitor referenced in the question seems to use DisplayPort, not Thunderbolt. There is no such limitation for DisplayPort, as that does not use PCI-E lanes at all. Even the 2016 MBP should be able to drive a 4K display from the USB ports on either side. – lxgr Jan 09 '20 at 13:40
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Same issue here, I bought a 2018MBP 512GB/16GB/I7 version, when I connect to my LG 4K (27UK850) display on the right (using equipped USBC to C cable), it only output 1920*1080 30Hz, the mouse and window is lagging with shadow, it is killing me, while my previous model 2016MBP 512GB/8GB works fine with this display, it can output 60Hz from any of 4 TB3 ports.

The fact is they are the same OS as I restored the entire OS using Time Machine. Interesting thing is, I called Apple, the operator said 2018 model has a spec bump on GPU, which requires HDMI 2.0 cable to get 4K 60Hz, she said changing a suitable cable is the only way, and I tried, it really worked, but in that case I need to connect my MBP to power using additional cable, rather than one USB C to C cable nails it all (power, image, audio).

Now I am connecting to the left side TB3 port just like you, and 40K 60Hz it is, I sent an email to that operator and ask her for an official reply on why same TB3 ports works differently, since I am AC+ user, I think she will get back to me real quick, I will update then.

Kae Tuk
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  • This certainly sounds like the same issue, good to have some corroboration - thanks for investigating, Kae. Hoping we will have some clear answers from Apple. – Daniel Jones Jul 30 '19 at 22:10