macOS: Why do scaled resolutions affect performance?

The resolution of a 4k display is 3840x2160. macOS can render at this resolution natively, but controls are much too small, and no sharpness/retina improvements are visible. The default solution is to render all UI elements at twice the size along each axis. This looks like 1920x1080 (which is 3840x2160 halved), but controls are large and text is crisp, because every “effective” pixel is using 4 pixels on the monitor.

Scaled resolutions lie between the bounds of these neat multiples. For example, the second largest “effective resolution” macOS provides for my display is 3360x1890. macOS renders itself at twice this resolution (6720x3780) and downscales to 4k. In addition to some UI blurriness (although this is barely noticeable), this naturally causes a performance hit because on every frame, macOS is:

  1. Rendering at a significantly higher resolution (~4 million extra pixels) than native 4k.
  2. Scaling the rendered frame down to 4k.

If the performance hit is unacceptable, options are limited to:

  1. Use an eGPU to handle the extra computation.
  2. Use a smaller monitor so controls don’t look too large in the default retina mode.
  3. Use a larger 4k monitor so the native 4k mode is usable without controls being too tiny.
  4. Use a scaling mode where the multiple is closer to the native 4k resolution so the performance hit is smaller (a scaled resolution of 2560x1440 only needs to render ~900k extra pixels per frame).

Verify the resolution macOS is rendering at by taking a screenshot (⌘⇧3) and examining its dimensions.