Resource preloading is a technique that tells the browser which files are most important for displaying the page, so it downloads them preferentially. It mainly concerns the main image displayed above the fold, critical styles and fonts, whose late loading causes text to flicker. The opposite technique is deferring scripts that are not needed for the first render. The right ordering can significantly improve perceived speed and the user experience score without changing the amount of data transferred. However, it is only worth preloading a few genuinely critical files – if too many resources are marked as priority, the effect is lost and loading slows down. The setup is verified by measuring before and after the change.
See also: Core Web Vitals, Loading states, Lazy loading.
