Quantifying User Experience
For a long time, "fast" was subjective. Engineers argued about which metrics mattered. Google changed this in 2020 by introducing Core Web Vitals (CWV) — a standardized set of metrics that quantify three key aspects of a healthy user experience. They are significant not just as engineering benchmarks, but because Google uses them as a ranking signal in search results, making them a business concern as well as a technical one.
The three pillars map to three fundamental user questions:
| Pillar | User Question | Metric |
|---|---|---|
| Loading Performance | Is it happening? | LCP — Largest Contentful Paint |
| Visual Stability | Is it usable? | CLS — Cumulative Layout Shift |
| Interactivity | Is it responsive? | INP — Interaction to Next Paint |
Each metric has defined "Good," "Needs Improvement," and "Poor" thresholds. Google recommends targeting the 75th percentile of page loads — meaning 75% of your users should be hitting the "Good" threshold, not just the average user.
The 75th percentile target is a deliberate choice. Optimizing for the median (50th percentile) means ignoring the worst quarter of your users — often those on slower devices or networks who are already the most frustrated.