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.
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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.
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Unfortunately there isn't a straightforward way of doing it. You've to go thru the files and adapt it to work with the LazyLoading script of your choice.
Anyway, I usually use this plugin https://github.com/verlok/vanilla-lazyload in case you want to give it a try.
I've checked your store and if there's "lazyloading" installed it isn't working at the moment.
It depends on how experienced you're with Shopify themes. My suggestions is to hire a pro IF you're not comfortable with messing around with code.
Because a part from installing the JS script, you've to also go thru all the files and change how images are loaded (like explained here)
Let me know if you need further help with this.