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.