Core Web Vitals failures happen when your site exceeds Google's performance thresholds: Largest Contentful Paint over 2.5 seconds, First Input Delay over 100ms (or Interaction to Next Paint over 200ms as of March 2024), or Cumulative Layout Shift above 0.1. These metrics measure real user experience, so fixing them requires addressing actual performance bottlenecks, not just passing synthetic tests. Start with LCP, which typically fails due to oversized hero images, slow server response times, or render-blocking CSS. Compress and serve images in WebP or AVIF formats, implement lazy loading for below-fold content, and use a CDN to reduce TTFB. We've seen LCP drop from 4.2s to 1.8s just by switching a 2MB hero image to a properly sized WebP version and preloading it with a link rel tag. For FID and INP, the culprit is usually JavaScript execution blocking the main thread. Defer non-critical scripts, break up long tasks into smaller chunks, and eliminate third-party scripts that don't directly serve users. Tag managers, chat widgets, and analytics often add 500ms+ of interaction delay. Audit them ruthlessly. CLS failures come from layout instability: images without width/height attributes, ads or embeds that load late and push content down, or web fonts causing FOUT. Reserve space for dynamic content with explicit dimensions, use font-display: swap cautiously, and avoid injecting content above existing elements. Google Search Console and PageSpeed Insights show field data from real users, which matters more than lab scores. A site can score 95 in Lighthouse but still fail field CWV if actual visitors have slow connections or old devices. We typically run fixes on a staging environment, verify with Lighthouse and WebPageTest, then monitor Search Console's CWV report for 28 days to confirm real-world improvement. If you're managing a 500-page site, prioritize templates over individual pages—fixing your product page template solves hundreds of URLs at once.