Core Web Vitals matter because Google uses them as ranking signals and because they directly correlate with user behavior. Pages that load slowly, respond sluggishly to clicks, or shift content around lose visitors and conversions. Google made these three metrics—Largest Contentful Paint, First Input Delay (now Interaction to Next Paint), and Cumulative Layout Shift—official ranking factors in 2021 specifically because they measure what frustrates real users. The business impact shows up in two ways. First, ranking drops: if your competitor's site loads in 1.8 seconds and yours takes 4.2, Google may favor theirs in close calls, especially on mobile. Second, engagement drops: we've seen bounce rates climb 15–25% when LCP crosses 3 seconds, even when the site eventually loads fine. Users just don't wait anymore. The tradeoffs get tricky with heavy sites. E-commerce platforms with large hero images, review widgets, and third-party scripts routinely struggle to hit the "good" thresholds (LCP under 2.5s, CLS under 0.1). You end up choosing between feature-rich pages and clean vitals. At Ottawa SEO, we typically optimize the highest-traffic landing pages first—homepage, top category pages, key product pages—rather than chasing perfect scores across 500 URLs. What kills Core Web Vitals most often: unoptimized images, render-blocking JavaScript, aggressive ad networks, and cheap shared hosting. A single unoptimized 2MB hero image can wreck LCP. Layout shift usually comes from ads or embeds loading late and pushing content down. The threshold that matters is "good" on at least 75% of page loads in Chrome User Experience Report data. That's real-world performance from actual visitors, not lab tests. You can have a perfect Lighthouse score and still fail field data if your hosting chokes under traffic or mobile networks are slow. Focus on real users, optimize the critical rendering path, and accept that some trade-offs—like keeping that testimonial carousel—might cost you a few points but still be worth it for conversions.