Principle of resolution scaling via External monitor (GPU performance)
It is not clear to me how your ranking is supposed to function.
You need to make a distinction between whether or not you're using HiDPI-mode (&quot;Retina-mode&quot;). This is very important.
You can run a 4k monitor at 4k resolution (as seen by the monitor) in two modes - either HiDPI Retina or native 4k. The HiDPI mode is the default chosen by macOS. In this case it will be labelled as &quot;Looks as 1080p&quot;.
When running in &quot;native 4k&quot; everything is rendered 1:1. This means that user interface elements would typically look very small on a normal sized monitor.
When running in &quot;HiDPI&quot; mode, you'll see that it will be labelled &quot;Looks like 1080p&quot;. I.e. text and user interface elements are the same physical size on the monitor as had you chosen a native 1080p resolution. However, text, video, pictures, etc. are actually rendered at 4k (3840x2160) resolution, giving you crisper text and full resolution video.
Whether or not &quot;native 4k&quot; mode or &quot;HiDPI mode&quot; is more taxing on the GPU depends on what applications you're running. If you're looking at a blank desktop, the &quot;native 4k&quot; mode should use the fewest resources on the system as a whole.
If you choose a different resolution, like for example 1440p in HiDPI mode, the system will actually render at double the size - i.e. 5120x2880 - and then scale it down to the 4k output for the monitor. In this case the system has to perform more work than it did for the &quot;Looks as 1080p&quot; HiDPI mode.
However, if you run 1440p in native mode, i.e. the output signal for the monitor is 1440p - then that is usually less taxing for the system than 4k native.