Preloading Galleries and Albums

I have not installed the plugin yet, but had a question. If I have multiple galleries/albums on a single page. How does preloading the images work? I will have it setup for a single thumbnail for each gallery, then the user clicks the thumbnail to see images in a lightbox. Are the images downloaded at that time? Or are they loaded on page load? Just don’t want to slow down the page with nice large photos, that’s all. Thanks! Can’t wait to use this plugin.

https://www.ads-software.com/plugins/photo-gallery/