Lighthouse flags 'efficiently encode images' on my site. What does it want?

How this differs from the next-gen formats finding

The next-gen formats finding says: switch JPEG to a newer format. This one says something else: even in the same format, your image is stored wastefully. The report compares each image with its expected size at a standard compression level, and if the gap is noticeable it lists the image with an estimated saving. So the fix is a lower quality in the same format, with no need to change the links on your site. The name and placement of this finding can vary between report versions.

Example: an image exported at maximum quality

One image like that at the top of your home page can outweigh the rest of the page put together. Yet the visual gap between quality 100 and quality 80 is hard to spot in a photograph on an ordinary screen.

Fixing it, step by step

1. Open the report and note the image names listed under this finding, along with the estimated saving for each.
2. Get the original versions of those images. If all you have is the uploaded copy, download it from your site.
3. Add them to the Image compressor, choose JPEG so the links keep the same extension, and start at quality 80.
4. Check the preview, especially banner images with text, and raise the quality to 85 if letters look hazy.
5. Download the files and rename them back to their old names if you will replace them on the server directly, because the tool adds -compressed to the name.
6. Upload them over the old ones, clear the site's cache if it has one, and run the report again.

If an image stays in the report

• Make sure the site is really serving the new file, not an old copy from a cache or a content delivery network.
• If the problem is dimensions larger than the display space, that is a different finding, fixed by resizing with the Image resizer.
• If the image is a photographic PNG, converting it to JPEG or WebP in the compressor saves a lot, but its extension changes and you must update the link.