Overview

We offer a variety of filters to get images down the wire and rendered as fast as possible. All of these can be enabled individually, or all at once using 'IISpeed rewrite_images'. IISpeed will back down when it detects that optimizations did not result in an image reduction.

User agent specific optimization

For user agents that support it, IISpeed is able to convert images to lossy or lossless WebP format for even smaller and faster downloads. For mobile user agents, IISpeed is able to first send low-res placeholders of images down the wire to speed up the initial view. Afterwards, a higher res version will be displayed.

Lazyloading

Part of the content prioritization options, with image lazyloading only images that are above the fold will be downloaded and rendered. Images that are below the fold, will be deferred and loaded by javascript inserted by IISpeed when they scroll into view. When combined with other content prioritization filters, like "prioritize critical css" and "defer javascript" , this can result in a very snappy web experience.

Recompression

The recompression feature will perform a few passes using different compression settings to figure out the smallest possible size for the configured image quality settings.

Transcoding

The recompression feature can also be configured to convert png to jpeg. Both jpeg and png can be configured for automatic conversion to webp. WebP will only be served to browsers that support it, and offers an additional 30 to 35 percent reduction in image size at the same quality as jpeg.

Progressive

By enabling 'convert_jpeg_to_progressive', recompression will trancode jpeg images that are not already progressive and larger then 10kb to progressive. Progressive jpegs allow better compression opportunities, as increase the speed at which browsers can show the user an initial image, resulting in a speedier experience.

Stripping metadata

Often, images still contain lots of metadata like icc profiles, previews, and copyrights. IISpeed can automatically detect and remove these.

Inlining

Small images can be inlined, to reduce the amount of requests needed to render the page. The threshold for this can be configured.

Inserting image dimensions

To help the browser render the page, IISpeed can insert the original image dimensions into IMG tags. Doing so prevents page reflows.


Filters marked bold are in the core filter set, which is enabled by default.

Related filters