Sorry for the delay -- waiting for a response from my friend. I had him set the Dell P2718Q to 4K (3840x2160) and move the mouse and drag some windows around. No discernible lag and certainly no ghosting. In the Graphics/Displays system report the monitor shows 3840 x 2160 @30Hz. He doesn't use it at this resolution -- everything is too small.At 60Hz?
I have a 2014 15-MBP with integrated graphics, and driving my Dell 2715Q is borderline unusable. Wishing an such as CS6 or Camtasia or Powerpoint, it works OK. But dragging things around, it ghosts and lags.