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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.

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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.

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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 2 at 18:56

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