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.


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.

  • 2
    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 2 at 18:56

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.

  • 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 at 22:10

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