Key takeaway: Performance metrics measure different aspects of the user experience, and the metric you optimise determines what experience you deliver — optimising a metric that does not reflect what users actually experience can improve the number while leaving the real experience unchanged or worse.
Metrics Measure Different Things
Performance is not a single number — it is a collection of metrics, each measuring a different part of the experience. Some measure how quickly content appears, some measure how quickly the page becomes interactive, some measure how stable the layout is while loading, and some measure how quickly the user can actually use the page. These are genuinely different aspects, and a page can be good on one and bad on another.
| Metric | What it measures |
|---|---|
| First contentful paint | When the first content appears |
| Largest contentful paint | When the main content appears |
| Time to interactive | When the page is usable |
| Cumulative layout shift | How much the layout shifts while loading |
A page can have fast first contentful paint — content appears quickly — while being slow to become interactive, or it can load quickly but shift its layout so much that users click the wrong things. Each metric captures a different part of the experience, and none of them alone tells the whole story.
The Trap of Optimising the Wrong Metric
The trap is optimising a metric that does not reflect what users actually experience. For example, optimising first contentful paint by showing a placeholder quickly can improve that metric while the real content — what the user actually wants — is still slow to appear, so the number improves while the experience does not. Similarly, optimising a metric in a way that worsens another can trade one problem for another.
This is why it matters to understand what each metric actually measures and to optimise the ones that reflect the experience users actually have, rather than optimising whatever number is easiest to improve or whatever metric happens to be reported.
The Core Web Vitals as a Starting Point
The Core Web Vitals — largest contentful paint, interaction to next paint, and cumulative layout shift — were chosen because they measure the aspects of the experience that matter most to users: how quickly the main content appears, how responsive the page is to interaction, and how stable the layout is. They are a good starting point because they focus on the user experience rather than on arbitrary technical numbers.
The point is not that these three metrics are the only ones that matter, but that they are a useful, user-focused set, and that the principle behind them — measure what users actually experience — is the right way to think about performance metrics generally.
The Bottom Line
Understand what each performance metric actually measures and optimise the ones that reflect the experience users actually have, since different metrics measure different aspects and optimising the wrong one can improve a number while leaving the experience unchanged or worse. Use the Core Web Vitals as a user-focused starting point, and be aware that optimising one metric can trade it for another, so the goal is the overall experience, not any single number.
