Sluggish mouse cursor with 28" 4K display I got a new MG28UQ 28" 4K monitor and I plugged it into my mid 2015 15" retina MBP using a mini display port to HDMI cable. The monitor itself reports it's receiving image at 60Hz, however, Settings > Displays only lists 30Hz under "Refresh Rate". If I lower down the scaled resolution to 1600 x 900, then it starts using 60Hz and the mouse problem goes away.

I'm using an Insignia Mini DisplayPort to HDMI cable that states it can handle 4K.

What am I missing?
Any scaling that is not exactly half of full resolution will introduce strain on the GPU and cause sluggishness. So on a 4K monitor your choices are either 1080p or 4K. I find neither acceptable. If you want 1440p (looks the best imho) you have to purchase a less crisp 1440p monitor or drop the big dollars for a 5K monitor and then run it at crisp retina 1440p which is exactly half of 5K and will not strain the GPU.