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 <img> 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.
Welcome to the Shopify Community!
This is Syed Mustafa a professional Shopify Developer. As I'm sure you well know, page speed has become a factor for ranking in the search engines. That's why if you're investing lots of time and money into getting the rankings you deserve, you shouldn't be blowing it by having a slow website.
Longer load times have also been shown to negatively affect conversions.
Drop your reply at Syedmustafanadeem99@gmail.com and let's start this project as soon as possible.