Core Web Vitals are Google's three key metrics for evaluating how real users experience your page's loading performance, responsiveness, and visual stability. They measure actual user interactions captured through Chrome User Experience Report data, not synthetic lab tests. Largest Contentful Paint (LCP) tracks how long it takes for the largest visible content element—usually a hero image, heading block, or video—to render in the viewport. Good LCP is under 2.5 seconds, needs improvement between 2.5-4 seconds, and poor above 4 seconds. This directly impacts perceived loading speed. First Input Delay (FID) measures the time between a user's first interaction—clicking a button, tapping a link—and when the browser can actually respond to that action. Good FID is under 100 milliseconds, needs improvement between 100-300ms, and poor above 300ms. Heavy JavaScript execution typically causes bad FID scores. Note: Google is replacing FID with Interaction to Next Paint (INP) in March 2024, which measures responsiveness throughout the entire page visit, not just the first input. Cumulative Layout Shift (CLS) quantifies unexpected layout shifts that occur during page load. Elements that move after initial render—images without dimensions, late-loading ads, web fonts causing text reflow—create poor user experience. Good CLS is under 0.1, needs improvement between 0.1-0.25, and poor above 0.25. At Ottawa SEO, we treat Core Web Vitals as competitive advantages rather than checkbox items. A site passing all three thresholds for 75% of real-world page loads gets a ranking boost, but the business impact comes from reduced bounce rates and improved conversions. We've seen e-commerce clients gain 15-20% more revenue just by fixing CLS issues that caused users to accidentally click wrong buttons. The metrics matter because they correlate with actual user frustration, not because Google says so.