A Core Web Vitals assessment failed means one or more of Google's three key user experience metrics fell below acceptable thresholds based on real-world Chrome user data. The specific failures break down like this: Largest Contentful Paint over 2.5 seconds (slow main content loading), First Input Delay over 100ms or Interaction to Next Paint over 200ms (laggy interactivity), or Cumulative Layout Shift above 0.1 (elements jumping around as the page loads). Google requires 75% of page visits to meet these thresholds for a passing grade. You'll see this failure in Google Search Console's Core Web Vitals report, PageSpeed Insights, or the Chrome User Experience Report. Search Console groups URLs by similar templates and flags entire clusters as poor, needing improvement, or good. A failed assessment doesn't immediately tank your rankings, but since June 2021's Page Experience update, it's a confirmed ranking factor that matters more when multiple pages compete for the same query. The most common culprits we see at Ottawa SEO are unoptimized images causing slow LCP, render-blocking JavaScript delaying interactivity, and ads or dynamic content injecting late causing layout shifts. WordPress sites with heavy page builders frequently fail CLS, while e-commerce product pages often struggle with LCP from multiple image carousels. To fix a failed assessment, start with field data in Search Console to see which metric failed, then use PageSpeed Insights lab data to diagnose specific resources. Prioritize fixes that address the worst metric first. For LCP, optimize your hero image with WebP format, proper sizing, and fetchpriority attributes. For INP, defer non-critical JavaScript and minimize long tasks. For CLS, reserve space for ads and images with explicit width/height attributes. We run Core Web Vitals audits across our 500+ domain portfolio monthly because mobile thresholds are stricter and Canadian hosting latency to U.S. users can push borderline pages into failure. Testing on real devices with throttled connections reveals issues lab testing misses.