There are several reasons why Google PageInsights might report than a website with compressed product images still needs image optimization. Here are the most common:
- Only targeting the homepage: When you test with Google PageInsights, it is usually the home page being tested. To test the effect on product page, please make sure a URL of a particular product page is submitted for testing. We recommend testing a product page before optimizing, and then again after the optimization.
- The image resolution is too high: If your product pages are still loading slowly, you may need to reduce the image resolution. For example, if the images are 2000x2000 pixels you can resize them to 1000x1000 pixels. The new file size should be about 75% smaller than the original file.
- Your theme generates large thumbnails: Google's PageInsights looks at thumbnails to determine whether they are larger than necessary. This has nothing to do with compression - it is determined by your theme when it automatically generates thumbnails. Therefore even if you shrink the images and reduce your loading speed, it will still detect a large resolution and dock you points. All responsive themes suffer this to some extent, although some themes are better at it than others.