Does ShortPixel Image Optimization plugin add a lazy loading effect to the compressed images?
The ShortPixel Image Optimization plugin for WordPress doesn't add a lazy loading effect to the webpages. What you are noticing is related to the fact that some of the optimized images are progressive.
Progressive images are images that load gradually. When the connection is slow, the gradual loading becomes visible.
Why are the images progressive?
Because when we optimize an image we choose between the progressive and the baseline version, depending on how small the final image is. If the progressive version takes less space on disk, then we save the image as progressive.
A rule of thumb is: larger images are progressive images while the smaller thumbnails are baseline images.