Retina Prefs for MacPerformanceGuide

This site serves “Retina-class” images for high resolution displays such as the Mac Book Pre Retina, iMac 5K, iPhone, iPad, high-res android phones, etc.

Retina grade images are served automatically after the first HTML page is served (a cookie is set so the server can know if the visitor has a retina display).

Retina handling: Retina AUTO (default) | Retina OFF | Retina ON

In some cases it can be desirable to force retina images off or on:

Connection speed—retina images are ~3X the size of regular images, hence forcing off retain can be useful for a slow connection.
iPhone and similar phones benefit only to a limited extent (depending on screen size and image size relatie to screen orientation). For for faster page-load times, force retina off.
Traditional non-retina computer screens look slightly soft with retina-resolution images, so retina images are not advised. But see next point.
Users who regularly zoom-up web pages on a non-retina display will find that retina images look sharper and clearer. See previous point.

Let your own eyes be the judge.