Crystal Ball checks covered

MOBILE_LCP_SLOW, FCP_SLOW, PAGESPEED_WEAKNESS

What this warning means

Crystal Ball should treat slow mobile LCP as a first-screen performance problem. The largest visible text block or image is arriving too late for mobile users.

  • A page can score perfectly in a local Lighthouse run but still show weaker PageSpeed Insights results if the public server, cache, or mobile test conditions differ.

Common causes

Common causes include slow initial response, render-blocking CSS, oversized hero images, late-loading fonts, too many third-party scripts, unoptimized background images, and cached old assets still being tested by PageSpeed Insights.

  • Hero images should use responsive WebP where possible.
  • Critical CSS should be lean.
  • Nonessential scripts should not block the first screen.

How to fix it

Start with the largest visible element on mobile. Make sure its image is correctly sized, compressed, and prioritized only if it is above the fold.

  • Use width and height attributes or stable CSS aspect ratios.
  • Preload only the true LCP asset when needed.
  • Defer nonessential scripts.
  • Trim unused CSS and avoid loading large libraries for small effects.
  • Set cache headers intentionally for assets and HTML.

How to verify

Compare browser Lighthouse, PageSpeed Insights mobile, and a repeated scan. If only PageSpeed is slow, check live public caching and whether the tested URL is receiving the same optimized assets.

  • Inspect the LCP element.
  • Check total blocking time.
  • Review cache headers.
  • Test after cache expiry or deploy version bumps.

FAQ

Why is PageSpeed slower than browser Lighthouse?

PageSpeed Insights runs from Google's test environment with simulated mobile conditions and public network access. Local Lighthouse may benefit from local cache, faster machine resources, or different timing.

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.

Need a second set of eyes?

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