Crystal Ball checks covered

MOBILE_LCP_SLOW, FCP_SLOW, PAGESPEED_WEAKNESS

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What this warning means

Largest Contentful Paint measures when the largest eligible content element in the initial viewport finishes rendering. For field data, an LCP of 2.5 seconds or less at the 75th percentile is considered good; a lab result describes only that test run.

  • A local Lighthouse run and PageSpeed Insights can disagree because the device, network, cache state, test location, and public server response are not identical.

Common causes

LCP time can accumulate in four broad stages: initial server response, delay before the resource starts loading, resource load time, and delay before the browser renders it. The useful fix depends on which stage is consuming the time.

  • Large or poorly prioritized hero images can delay image-based LCP.
  • Render-blocking CSS or fonts can delay text-based LCP.
  • Client-rendered content and heavy scripts can postpone discovery or rendering.

How to fix it

Identify the actual LCP element and its timing breakdown before changing assets. Optimize the element that wins the mobile test, then remove delays in front of it.

  • Serve an appropriately sized, compressed image with stable dimensions.
  • Prioritize the true above-the-fold LCP resource without preloading everything.
  • Keep essential text and image references in the initial HTML when practical.
  • Reduce render-blocking CSS and defer nonessential scripts.
  • Keep font loading from delaying a text-based LCP element.

How to verify

Run several comparable mobile tests and inspect the LCP element, subpart timings, and request waterfall. Treat one unusually fast or slow result as a clue, not a verdict.

  • Confirm that each run identifies the expected LCP element.
  • Separate server delay from resource and render delay.
  • Check the public response and cache headers.
  • Retest after the deployment and cache version are verifiably live.

FAQ

Why is PageSpeed slower than browser Lighthouse?

PageSpeed Insights includes a standardized remote lab test and, when enough data exists, a field-data section based on real Chrome visits. Local Lighthouse uses your machine, network, settings, and cache state, so the numbers are not directly interchangeable.

Should I build a separate mobile site?

Usually no. A responsive page with correctly sized assets, lean CSS, and deferred scripts is usually better than maintaining a separate mobile-only site.

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