In the case of your theme, the images don’t have lazyloading, and to make it a bit more tricky, all the sections display the images as background images, as opposed to using an tag.
Applying lazyload to background images is a bit more complicated then applying it to image tags, its not serious coding, its straight forward , but I imagine it might be confusing to do for someone with little coding experienced.
Maybe someone else on the forum can recommend a lazy loading plugin that makes it easy to set up lazyloading for background images, I’m only familiar with lazysizes.