When Google reports a Core Web Vitals assessment failure, it means your page scored "Poor" on at least one of the three metrics using real Chrome user data over the previous 28 days. The thresholds are strict: Largest Contentful Paint above 4 seconds is poor (good is under 2.5s), First Input Delay above 200ms or Interaction to Next Paint above 200ms is poor (good is under 100ms), and Cumulative Layout Shift above 0.25 is poor (good is under 0.1). Google evaluates at the 75th percentile, meaning if your slowest quarter of users exceed these limits, you fail. Most failures we see at Ottawa SEO come from LCP and CLS issues. LCP failures typically stem from unoptimized hero images, render-blocking CSS and JavaScript, or slow server response times. A 3MB hero image on a shared host will blow past 2.5 seconds easily. CLS failures usually come from images or ads without explicit dimensions, web fonts loading late and causing text reflow, or dynamically injected content pushing existing elements down the page. The assessment uses field data from real visitors with actual devices and network speeds, which is harsher than lab tests in PageSpeed Insights. A page might score 95 in Lighthouse but fail the field assessment because your audience skews mobile on slower connections. This is common for service-area businesses in Ontario where mobile traffic dominates but LTE coverage varies. To fix a failed assessment, prioritize the metric dragging you down. For LCP, compress images to WebP under 200KB, implement lazy loading for below-fold content, and use a CDN or upgrade hosting if TTFB exceeds 600ms. For CLS, set width and height attributes on every image and reserve space for ads. For INP, defer non-critical JavaScript and break up long tasks. Changes take 28 days to fully reflect in the Chrome User Experience Report, so fixes aren't instant. We track both field data in Search Console and lab data in PageSpeed Insights to catch regressions before they hurt rankings.