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Check image optimization in PageSpeed Insights, then verify what the browser actually downloads. PageSpeed Insights (PSI) combines real-user field data with Lighthouse lab diagnostics; Lighthouse flags oversized images and older formats, while the browser’s HTML and network panel reveal whether responsive variants and fallbacks are working. Finally, compare performance before and after changes using LCP, CLS, and INP—not just a Lighthouse score.
1. Start with PageSpeed Insights
- Open Google PageSpeed Insights and enter the exact page URL you want to assess.
- Review the field data and lab data separately. PSI provides both: field data is based on a trailing 28-day collection period of Chrome user experience measurements, while Lighthouse lab data is a controlled simulation intended to help diagnose issues. A page may have field data for one device category and not another, or no field data at all; do not treat missing field data as proof that the page is fast or slow.
- Check the mobile and desktop results where available. If field metrics are unavailable, use lab findings to identify likely implementation issues, then validate with real-user data after deployment.
Field data describes user outcomes over time; lab data helps reproduce and investigate a page under a consistent test. They answer different questions, so one should not be substituted for the other. See PSI’s explanation of lab and field data.
2. Read Lighthouse’s image findings
From PSI, open the Lighthouse results and inspect the image-related opportunities and diagnostics. You can also generate a Lighthouse report through Chrome DevTools, the command line, or as a Node module; its audits point to specific opportunities, but a passing audit is not a guarantee that every image is ideal for every device or use case. See Lighthouse documentation.
Look for oversized images
Lighthouse compares an image’s rendered dimensions with the downloaded image’s dimensions. Its oversized-image check flags an image when the rendered version is at least 4 KiB smaller than the actual file. That is a useful clue: the browser may be downloading more pixels or bytes than the displayed context needs. Open the affected URL and inspect its rendered size, intrinsic dimensions, and downloaded bytes before deciding how to change it. The audit threshold and image-delivery guidance are described in Chrome’s image-delivery guidance.
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Look for older image formats
The image-format audit identifies eligible BMP, JPEG, and PNG files and estimates savings from WebP conversion and AVIF sizing. It omits an image when estimated savings are below 8 KiB, so an absent opportunity does not mean every image is already optimal. Treat estimates as leads to check rather than guaranteed production savings. The format audit details are in Lighthouse’s image-format guidance.
3. Confirm what the browser serves
Audits suggest fixes; the page itself shows whether they are implemented. In Chrome, open DevTools, select Network, reload the page, and filter to images. For important image requests, inspect the URL, response size, dimensions, and format. Compare those requests with the element’s rendered dimensions in the Elements panel.
Check responsive variants
Inspect the image markup for srcset and sizes. These let the browser choose among image candidates according to the rendered context and viewport. Confirm that the selected candidate is suitable for the actual display size: the presence of several variants alone does not show that the right one was chosen. An image CDN may perform a similar job by transforming and delivering variants for viewport dimensions. Do not resize every image to a single universal size; generate sizes appropriate to the contexts where images appear. Chrome explains responsive image delivery at its image-delivery page.
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Check format delivery and fallback
WebP and AVIF can offer better compression than older JPEG and PNG formats. AVIF support is more limited, so check the actual formats and fallback behavior for the browsers your audience uses. A common approach is a <picture> element with modern formats as sources and a JPEG or PNG fallback in the <img> element. Verify the request in the Network panel rather than assuming that markup or a conversion tool ensures all visitors receive the intended file. See Chrome’s format guidance and responsive delivery guidance.
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Image efficiency matters most when it changes user-visible loading or layout behavior. Use PSI’s field metrics to judge real-user outcomes and lab diagnostics to investigate likely causes. Google’s current Good thresholds are:
| Metric | Good threshold | Image-related question |
|---|---|---|
| LCP (Largest Contentful Paint) | ≤ 2,500 ms | Is a large above-the-fold image slow to discover, download, or render? |
| CLS (Cumulative Layout Shift) | ≤ 0.1 | Does the page reserve space for images before they load? |
| INP (Interaction to Next Paint) | ≤ 200 ms | Does image-related work contribute to sluggish interaction? Check the broader page as well; this metric is not an image-size audit. |
Thresholds and metric interpretation are documented in Web Vitals and PageSpeed Insights documentation. For LCP, inspect the largest visible content and whether its image is discovered and delivered promptly. For CLS, make sure image dimensions or an aspect ratio reserve the intended space. Recheck field metrics after deployment: a single lab run is not evidence of a sustained real-user improvement.
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5. Choose fixes based on the finding
- File is much larger than its rendered context: create appropriately sized variants and serve them with responsive markup or an image CDN. Avoid scaling a single large source down in the browser for every viewport.
- Large JPEG or PNG with meaningful estimated savings: test WebP or AVIF, preserve a fallback where needed, and compare the delivered output rather than relying only on the tool’s estimate.
- Hero or other above-the-fold image is implicated in LCP: prioritize checking its request, selected variant, and delivery. Do not apply a blanket loading or priority change without considering how that affects the page.
- Images shift content as they appear: provide intrinsic dimensions or an aspect ratio so layout space is reserved.
- WordPress site: Chrome recommends a plugin or service that automatically converts uploaded images to optimal formats. Check that the resulting page actually serves suitable variants and fallbacks.
Google names ImageMagick’s convert utility as one option for applying similar optimizations, but third-party transformations can sometimes make an already optimized image larger. Compare before and after bytes and visual output rather than assuming another conversion always helps. See Google’s image optimization guidance.
6. Recheck after changes
- Run PSI again on the same URL and device category. Compare the same types of results: field with field, and lab with lab.
- Use Lighthouse’s image audits to see whether the targeted delivery issues changed.
- Inspect the browser’s Network panel again to verify which variants and formats are actually requested and how many bytes they transfer.
- After the change has had time to appear in field data, assess LCP, CLS, and INP against the Good thresholds. A short-term lab difference does not by itself establish a real-user improvement.
7. Troubleshooting common mismatches
PSI shows no field data
Field data depends on available Chrome user experience measurements over the documented trailing 28-day window. If it is unavailable, use lab results for diagnosis and check again later; do not infer real-user performance from the absence of data.
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Lighthouse reports savings, but the page still looks optimized
The audit estimate is not a guarantee of savings under every delivery path. Inspect the exact downloaded resource, its bytes and dimensions, and any transformations performed by a CDN or proxy. A further conversion can increase an already efficient file’s size.
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The HTML has srcset, but a large file is still downloaded
Check the browser-selected request in Network and compare the sizes information with the image’s actual rendered context. Responsive markup only helps when candidate sizes and selection inputs correspond to the display layout.
A converted format is not served to every browser
Verify the request format for the browser being tested and provide an appropriate fallback when using AVIF, whose support is more limited. Test the fallback path rather than assuming all clients behave like the browser used for development.
An image tool produces a larger file
Compare the output bytes with the original and retain the smaller suitable asset. Google notes that third-party transformations can sometimes enlarge an already optimized image.
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Or skip the browser setup
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Does a good Lighthouse score mean every image is optimized?
No. Lighthouse audits identify issues and opportunities, but confirm the actual downloaded variant, format, and bytes in the browser.
Should I convert every image to AVIF?
No. Test whether conversion reduces delivered bytes and retain a suitable fallback for browsers with more limited AVIF support.
Can a screenshot tell me whether images are optimized?
A screenshot shows visual rendering, not transferred bytes or responsive request selection. Use it as a visual companion to PSI and the Network panel.
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