Most likely they meant asynchronous loading of images in the gallery. For example, could lead Lazy Load.

Lazy Load in Brief:

Images are usually the heaviest elements of a page. Therefore, if there are many images on a page, then you should not download them all at once. Firstly, it will speed up page loading, secondly, it can significantly reduce the load on the server, and thirdly, it will save traffic to the user.

Pros Lazy Load:

Delayed loading has many advantages. Instead of downloading all the content, it downloads only the part that is visible to the user in the browser window. This can be expressed in different ways, ranging from just delaying the loading of images and ending with full-scale endless scrolling of content.

High download speed site
Downloading occurs only when necessary. This feature helps to significantly increase the speed of the site as a whole. This is a great way to reduce the number of visitors who left your site due to slow loading.

Intuitive user interface
Visitors will no longer need to click on a new page and wait for it to load. They have the opportunity to calmly and naturally scroll the information until they find what they are looking for.

Minuses of delayed loading

SEO damage
Yes, we wrote that the speed will positively affect while scanning your site. However, there is a negative impact. Given that some content simply does not appear until the user reaches it, then when scanning, search engines such as Google may simply not see it. But this could happen with a fixed number of posts on the site. It simply means that the search engine cannot take into account some important information that exists below in your archives.

Poor performance on older browsers
Given that the content is loaded only when users scroll through the site before it, there may be problems with viewing the content due to performance, especially on older browsers or on a slow Internet. You may have noticed this while working on sites with deferred loading, if you want to quickly scroll down, then you have to wait a few seconds for everything to load. In addition, users will not be able to control their navigation through the content or use the browser's back button when searching for content that they have recently viewed.

When to use deferred loading

As you can see, the advantages and disadvantages of deferred download fall into the same areas: search engine optimization and user interaction. To get the most out of deferred loading, you need to use it sparingly. Try using it only on your home page or pages with a large number of images and turn it off on pages that host valuable SEO content. Experiment and verify your data using Google Analytics (for site traffic and SEO), Pingdom (for site speed) and just pay attention to user feedback.

PS As information, articles were collected from the Internet.