Sluggish mouse cursor with 28" 4K display
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.