Test a page in Google PageSpeed Insights (PSI), read its real-user field data separately from its Lighthouse lab diagnostics, then use Chrome DevTools to trace any image or Largest Contentful Paint (LCP) delay. For site-wide patterns, check Search Console’s Core Web Vitals report. This sequence helps distinguish an image that is too large from delays in discovering, requesting, or rendering it.
Test a page with PageSpeed Insights
- Open PageSpeed Insights and enter the full URL of the page you want to diagnose.
- Review the field data and note whether it describes that exact URL or the broader origin. PSI may show origin-level Chrome User Experience Report (CrUX) data when URL-level data is unavailable; the origin result is not a page-specific measurement.
- Read Core Web Vitals before focusing on the overall score. Google’s documented good thresholds are LCP at or below 2.5 seconds, Interaction to Next Paint (INP) at or below 200 milliseconds, and Cumulative Layout Shift (CLS) at or below 0.10. The corresponding needs-improvement and poor ranges are shown below; Google’s accessed guide does not state a publication year for these figures.
- Then inspect Lighthouse’s lab results and opportunities. Treat them as a controlled diagnostic run, not a replacement for field experience.
| Metric | Good | Needs improvement | Poor | What it indicates |
|---|---|---|---|---|
| LCP | 0–2,500 ms | 2,500–4,000 ms | 4,000 ms or more | How quickly the main content appears; it may be a hero image or a heading. |
| INP | 0–200 ms | 200–500 ms | 500 ms or more | How responsive the page is to user interactions. |
| CLS | 0.00–0.10 | 0.10–0.25 | 0.25 or more | How much unexpected layout movement occurs. |
Thresholds and metric definitions: Chrome for Developers’ PageSpeed Insights guide. Keep the field and lab sections distinct: CrUX summarizes real users’ experiences, while Lighthouse tests under controlled conditions.
Use lab results to narrow down an LCP delay
LCP identifies when the largest main-content element becomes visible, but it does not by itself explain why that took time. Compare LCP with First Contentful Paint (FCP) and inspect the timing evidence. A large gap between FCP and LCP can mean the LCP resource was not available early enough—for example, because JavaScript reveals or requests the image—or that browser work delayed rendering. Google’s LCP guidance recommends checking whether lab LCP broadly aligns with field experience.
Do not assume that image compression is the answer just because an image is the LCP element. Its request may start late, the server may respond slowly, or browser rendering work may hold it back. Use the breakdown to identify which part of the timeline is taking time.
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Trace image delivery in Chrome DevTools
- Open the page in Chrome and open DevTools. Select Performance, then record a page load or reload the page with the Performance panel open.
- Inspect the Performance Insights view for Improve image delivery. It can point to opportunities such as serving AVIF or WebP and using an image sized appropriately for its rendered dimensions.
- Open LCP breakdown to inspect the stages contributing to the LCP element’s timing.
- Check LCP request discovery for reasons the browser may take longer than expected to find or start fetching the LCP image.
- Review other insights on the same page, including document request latency and rendering or layout work. Fix the slow stage indicated by the evidence rather than applying image compression indiscriminately.
Chrome documents these image and LCP diagnostics in its Performance Insights reference. The exact recommendations depend on the page and the recorded run.
Find recurring problems across a site
Use Search Console’s Core Web Vitals report to identify groups of similar URLs classified as Poor, Need improvement, or Good for LCP, INP, and CLS. A group-level report can reveal that a template or page type has a recurring issue, but it does not replace investigating a particular page.
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- Open the property in Google Search Console and select Core Web Vitals in the experience reports.
- Review the affected metric and URL groups, then choose representative URLs to investigate.
- Run those URLs individually through PSI and, where needed, record them in DevTools to trace the page-specific cause.
Google explains the report’s URL-group approach in its Core Web Vitals report documentation. Site-level patterns and individual-page diagnostics answer different questions, so use both when applicable.
Repeat the test after a change
After changing image formats, dimensions, loading behavior, or rendering logic, repeat the same page-level checks under comparable conditions. Keep the tested URL and device or test conditions consistent when comparing lab runs. Then watch field data as it updates: it reflects aggregate real-user experience, not the result of one controlled run. A better lab result alone does not prove that every visitor’s experience improved.
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What each testing view can and cannot tell you
| View | Useful for | Limitation |
|---|---|---|
| PSI field data (CrUX) | Aggregate real-user experience for a URL, or an origin if URL-level coverage is absent. | URL-level coverage may be unavailable; real users have varied conditions. |
| PSI or DevTools Lighthouse lab audit | A controlled run with diagnostic opportunities, including image and LCP clues. | One lab run does not stand in for aggregate field experience. |
| Search Console Core Web Vitals | Patterns among groups of similar URLs and their metric status. | Group reporting does not diagnose the cause on a specific page. |
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For example, this cURL request saves a WebP screenshot of Stripe:
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Frequently Asked Questions
Why does PageSpeed Insights show field data for my origin instead of my URL?
PSI may not have URL-level CrUX coverage for the page and can show origin-level data instead. The report’s field-data section indicates which scope it provides.
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Does a good Lighthouse result prove my page is fast for all visitors?
No. Lighthouse is a controlled lab test; CrUX field data summarizes real users’ experiences. Use the lab to investigate and field data to understand aggregate experience.
Is image compression always the fix for a slow LCP?
No. The image may be slow to discover or request, or browser rendering work may delay its display. Inspect LCP breakdown and request-discovery evidence before choosing a fix.
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