Preloading of critical assets is a premium feature of WP-Optimize.
Some assets which are considered critical might be loaded later because they are included via a script or from within a CSS file.
A typical example are font files, background images, or scripts which are loaded from an other scripts. They will typically start loading once the other assets have been downloaded and parsed.
Preloading those assets will enable them to be loaded in priority, and could result in a faster page load.
Posted in: Minification