I am not at all an expert on page load time. But nowdays modern browser show quite nicely, where the time is "spent" on. Often a website is slow, because of many requests. Every image (non cached) means a reqeuest to your server. This can have serious negative effects on your page load time. To deal with it, you can use "Lazy Loading" Plugins. Another way - which I like more - is to defer images. Yesterday I found the following article:
IMO the artice is very easy to understand. If you want to implement it in your shop: You just need to adjust tpl files of your theme. The <script> part can be added in footer.tpl. The rest depends on what you want to achieve. As an example: https://www.spielezar.ch. The images on the footer (payment logo and facebook image) are loaded deferred. For the visitor there is no difference at all, as this images are not in the view when the page is loading. But the page just loads faster.
If you have any questions about it, feel free to ask!