By default, fancyBox fully preloads an image before displaying it. You can choose to display the image right away. It will render and show the full size image while the data is being received. fancyBox supports srcset with the w descriptor across all browsers. This allows to serve different scaled images based on viewport width. What this means is that mobile users can now be served smaller file sizes, which in turn speeds up the delivery of the content. Also over time this can save quite a bit of bandwidth, which in turn can save you money.