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lazysizes

By Patrick Sletvold

Performance & Optimization / Image Optimization & Compression

lazysizes Review - In-Depth Analysis

High performance and SEO friendly lazy loader for images, iframes and more. Features include low-res Blurhash placeholders and image fade-in.

Performance Overview

WPS Score

6.89/10

User Rating

5/5 (based on 6+ reviews)

Active Installations

600+

Update Frequency

Below Average It is calculated using the weighted average of same category plugins.

Security Score

1 / 10 (Very Low Risk) It is calculated using the weighted average of all versions.

What It Does

lazysizes is a WordPress plugin designed to enhance the performance of your website by implementing an SEO-friendly and auto-triggering lazy loader. It supports various elements including images, iframes, and scripts/widgets, making it versatile for different content types.

This plugin optimizes resource loading by distinguishing between crucial in-view and near-view elements, thereby improving perceived performance. Users can also utilize low-resolution or blurry placeholder images through the Blurhash algorithm for a smoother visual experience.

To use lazysizes, simply activate the plugin and adjust the settings as needed. It automatically replaces the src and srcset attributes with data-src and data-srcset, allowing for dynamic loading of content as users scroll through the page.

Other Notable Features

Here are a few other notable features of this free lazysizes plugin.

Lazy load

Performance

Blurhash

FAQ

Lazysizes filters images added to the page using the_content, post_thumbnail_html, widget_text and get_avatar. If your images are added using another function (wp_get_attachment_image for example), lazysizes does not filter them by default. There are several options for changing what lazysizes filters, like enabling it to filter acf_the_content for WYSIWYG content from Advanced Custom Fields, and enabling wp_get_attachment_image support (somewhat limited, see below). For unsupported use cases you can also filter the HTML yourself by passing it to get_lazysizes_html.

While this plugin has opt-in support for wp_get_attachment_image, it doesn’t add a no-Javascript fallback, which causes images to become invisible for users where Javascript is disabled or unsupported. We cannot fix this for you automatically, but you can fix this with a couple of changes to the code that uses wp_get_attachment_image. For example, if a theme has: echo wp_get_attachment_image($id);, changing it to the following would lazy load the image and add no-Javascript fallback if enabled in settings: echo get_lazysizes_html( wp_get_attachment_image($id) );

If a popular plugin is incompatible and has a filter for modifying the HTML output, lazysizes could most likely get support for that plugin. In that case, feel free to ask! If the plugin has no such way to filter the output, they would have to add one for lazysizes to leverage.

The low-res Blurhash placeholder feature generates a text string for each image using the Blurhash algorithm. This string includes all the information necessary for the Blurhash script running in the visitor’s browser to decode it into a blurry image, which will be shown while the real image is loading.

Because the final image placeholder is generated in JavaScript, users on faster internet connections can sometimes see the full image directly for images that are above the fold. Images lower down on the page will have a placeholder ready by the time the user reaches them.

The placeholder Blurhash string is not computed on page load, as it can in some cases take several seconds to do so. Instead, it will need to be pregenerated. As long as Blurhash is enabled in the settings, all new images uploaded will have a Blurhash string generated automatically. Additionally, you can manage the Blurhash string for each attachment individually in the Media Library. There is an option to generate and store Blurhash strings on page load, which can be used to easily generate Blurhash strings for lots of images by visiting the page they’re shown on. Just remember to turn that option back off, or your visitors may be slightly upset.

For technical reasons, Blurhash is only supported for local image attachments, and at the moment is not officially supported for picture elements. Images without a Blurhash string will behave just like they do with the option turned off. Blurhash placeholders work with the existing effects, like fade-in, but in some cases the perfect fade-in effect may not be possible. The Blurhash placeholder will still fade in, but it might not fade when transitioning to the full image. This is because of a few edge cases not supported by the advanced Blurhash reveal effect.

The advanced Blurhash reveal effect works by creating an additional image element positioned under the regular image. This gives the best result in combination with the fade effect, but might not support all WordPress themes. Safeguards exist to prevent using the advanced effect when not supported (by falling back to the slightly imperfect fade effect), but in some cases problems may still occur. If you see this type of problem, you can disable the advanced Blurhash reveal effect in the settings. Feel free to contact me on the support forums, and I may be able to work out what was going wrong.