External display resolution does not match what is specified in Mac OS settings

I am using Dell P2715Q display (4K) with MacBook Pro (Retina, 13-inch, Early 2015). It is by default scaled to 1920 x 1080 @ 60 Hz (copied from System Report). But the display shows "Resolution: 3840x2160, 60Hz" (at the bottom of the Menu). It bothers me cause I have some performance issues working with this display. I also have Dell U2715H display (2K) at home which doesn't show any performance problem at all. I assume the problem is in Mac OS scaling but don't see a way do diagnose or fix it.