Core Web Vitals exist to give Google and site owners a standardized way to measure real-world page experience. They quantify three critical moments users hate: waiting for content to load, clicking a button that suddenly moves, and tapping something that doesn't respond. Google made them ranking factors in 2021 because slow, janky pages drive users back to search results, which wastes Google's credibility. The three metrics are Largest Contentful Paint (LCP) measuring load speed, Cumulative Layout Shift (CLS) measuring visual stability, and Interaction to Next Paint (INP) measuring responsiveness. INP replaced First Input Delay in March 2024 because FID only measured the first interaction, missing ongoing sluggishness. Good thresholds are LCP under 2.5 seconds, CLS under 0.1, and INP under 200 milliseconds, measured at the 75th percentile of real user visits. The purpose extends beyond rankings. Sites that pass Core Web Vitals see measurably lower bounce rates and higher conversions because users don't abandon slow experiences. A Toronto e-commerce client we audited was losing 18% of mobile visitors to a 4.2-second LCP caused by unoptimized hero images. Fixing that directly improved revenue, and the ranking lift was secondary. Core Web Vitals also force transparency. Before 2021, Google's speed signals were opaque. Now you can see exactly what Google measures via Chrome User Experience Report data, which pulls from real Chrome users visiting your site. This levels the playing field because you're optimizing against the same dataset Google uses for rankings. For agencies running portfolio sites like we do, Core Web Vitals create a maintenance floor. You can't just publish and forget. A site passing vitals today can fail next month if you add a heavy plugin or third-party script. We treat vitals as ongoing QA, not a one-time optimization, because Google measures them continuously from actual visitor devices. That real-user measurement makes them harder to game than older lab-only metrics, which is exactly the point.