I think it's a Mac and an HDPI thing. I had to stop using emacs because it was too slow on my work MacBook Pro connected to a 4K screen. Scrolling was painful and only marginally better with nativecomp.
I am using emacs on a Mac, often with a 4K screen, and generally have no problems. I did have a scrolling lag issue a few months ago, but it was because of a bit of code that I had added to my modeline that was doing way too much work every time something changed in the buffer. The profiler pointed me right to it.