There might be a performance drop but no one has done benchmarks because the drop is not noticeable or the drop in visual quality is not worth the performance increase. If you're gaming then you might want to choose the display's native resolution for best quality/performance or a lower resolution (not HiDPI mode) for better performance. For a 4K display, a game should performance the same using "Looks like 1920x1080" or 3840x2160 native.
HiDPI modes are chosen by macOS, not the display. For a 4K display, the default modes will be HiDPI. You can get non-HiDPI modes by holding the option key and clicking "Scaled" in the Displays preferences panel, then selecting "Show low resolution modes".
SwitchResX can also show all the HiDPI and non-HiDPI modes in a list.