Core Web Vitals

Core Web Vitals are a standardised set of metrics intended to capture user experience rather than machine timings. The current three are Largest Contentful Paint (when the main content appears), Interaction to Next Paint (how quickly the page responds to input), and Cumulative Layout Shift (how much content moves around unexpectedly while loading).

Their value is that each maps to a felt experience, which makes them arguable with non-engineers. Their limitation is the usual one: they are proxies, and optimising a proxy directly diverges from optimising the experience — see Goodhart's Law. A page can score well and still feel bad.

Each has a characteristic cause. Largest Contentful Paint is usually dominated by the Critical Rendering Path and by images. Interaction to Next Paint is usually dominated by main-thread JavaScript, which makes it a Code Splitting and Hydration problem. Layout shift is usually caused by content arriving without reserved space — images without dimensions, fonts swapping, banners injected above existing content.

Field data (real visitors) and lab data (a synthetic run) frequently disagree, and the disagreement is informative rather than a measurement error: lab runs use a fixed device and network, and real users do not.