Differentiate between data sources
The lab test is run under specified conditions and helps diagnose a specific page. Field data reflects the experience of real users over a period of time and may not be available for a low-traffic URL. After changing the code, the laboratory result changes immediately, and the accumulated field data changes gradually.
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Tie the metric to the cause
For LCP, find the main content element and check its download and server response. For INP, explore slow interactions and long JavaScript tasks. For CLS, check the space for images, banners and fonts. Do not apply the same correction to all three metrics.
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Check important page templates
Choose the homepage, a landing page, a category, a product and a form. Record measurement conditions, changes made, and side effects. After optimization, check the availability of buttons, payment and analytics. Good metrics help the user, but do not guarantee high positions without relevant content and a technically understandable site.
Why do the numbers vary between phones?
The result is affected by device, network, cache state, third-party resources, and the specific scenario. Therefore, comparable conditions and multiple data sources are needed.