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I just got a nice 4K monitor (LG 27ud68p-b) and, when connected to my 2017 MBP 15" via USB-C to HDMI, the refresh rate is noticeably slow and results in choppy moving images. Even when it's not a very compute intensive image like just moving the cursor around, it's pretty bad.

When using USB-C to Displayport, however, things are snappy and look great. What is the difference?

FWIW - I use HDMI with this laptop and different monitors and they are great.

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Turns out, the white Apple USB-C to HDMI (plus power and USB-C) adapter only supports 4k @ 30hz. The USB-C to DisplayPort cable I have can successfully deliver 4k60 so that's why movements are fluid.

The other high-end monitors that I've successfully used this laptop+dongle+hdmi with are actually a little less than 4k so they still get the 60hz performance.

To be clear - there are different versions of HDMI and DisplayPort technologies. This includes the devices and also the cables/dongles that carry the signals:

https://en.wikipedia.org/wiki/HDMI#Refresh_frequency_limits_for_standard_video https://en.wikipedia.org/wiki/DisplayPort#Refresh_frequency_limits_for_standard_video

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