Lazy loading – what is it?
Lazy loading is simply a script entered into the code of the page causes that images and video files don’t load as soon as the users open the page, but only when they’re in this part of the website where the images or videos are located. It’s often used as part of technical SEO services optimization to improve a site’s performance. 
Currently, people pay more and more attention to the speed at which a website is loaded both on the computer and on the phone or tablet. Users and enthusiasts of social media, especially those in the form of mobile apps, may have come across lazy loading already two years ago while they were using Tumblr for Android and iOS.

Source: https://www.xda-developers.com/google-chrome-image-lazy-loading-speed-up-page-loading-mobile/
Lazy Loading SEO – Slower means faster
In its official guidelines, Google states that the loading speed of the website is a ranking factor affecting its SEO. Maybe it’s high time to think about it carefully and prioritize your online work so that you can improve your search results and customer satisfaction. We can distinguish several factors that influence the loading speed of the website. As you’re only a website owner, you don’t have control over such aspects as the quality of the user’s Internet connection. The only thing that you can bear in mind in this situation is that not everyone has a fast Internet connection and not every telephone network provides satisfactory bandwidth in every corner of the world. But, on the other hand, there are numerous factors that you can easily influence. If the website loads slowly, then the hosting server is first to be blamed. It must have sufficient performance so that the slowdown of the website isn’t noticeable for the users, especially when the website traffic increases. And the last factor is entirely dependent on you as the person managing the site because even the fastest server won’t be able to improve the loading speed of a heavy website. 
How does lazy loading affect SEO?
A fast website definitely allows you to increase the effectiveness of various SEO activities. Additionally, it doesn’t require investing any money and will help you to win users from both organic and paid search results. Here is a sample code using the lazy loading technique:
As soon as the user reaches the point on the page where the graphic is located, it’s loaded and displayed. Everything seems perfect, but there is one more very basic question – what about Google robots? Does lazy loading affect SEO in a direct way? Unfortunately, search engine bots prefer such websites that can be easily scanned using only HTML code. Nowadays Googlebot can properly crawl and render Javascript based on Chrome41 – but it often causes difficulties and challenges when it comes to indexing your website correctly. So while there will be no negative effects for the users, as the graphics will be displayed in the original size exactly when it’s needed, it may be a bit problematic for the robots as they may not notice it. However, there is a way to achieve a win-win situation. We fully recommend you to see the video prepared for webmasters by Google: https://www.youtube.com/watch?v=PFwUbgvpdaQ&feature=youtu.be&t=33m
How to deploy the lazy loading script?
As we’ve already mentioned, lazy loading is implemented by making changes in the code of the website. The code can be written by the programmer, nevertheless using ready-made CMS plugins like WordPress or Javascript is definitely an easier solution.
Lazy loading for mobile devices
According to Google, more than 50% of searches come from mobile devices so you can easily notice how much potential lies in this form of technology. However, the greatest risk of limited bandwidth concerns exactly mobile devices. Here, there is also no need to display images in high definition right after entering a website so it’s a good idea to also take this aspect into consideration during mobile optimization.
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Expert Comment
There’s a quite dispute in the SEO industry on how important role plays the website loading speed in the whole SEO process. I think it’s important to remember that search engine optimization is such a complex task to carry that there’s no way to definitely determine the value each factor brings to the final effect. Regardless of what impact loading time has on website optimization, it’s a fact that nowadays users’ won’t re-visit your site when it’s not pleasant to use. That’s, for me, the main reason to get the website speed elements really matters now – for the sake of UX.
Matt Calik
Delante CEO