主要なパフォーマンス指標

Performance metrics are how we measure fast versus slow — key indicators that quantify the experience during loading and interaction.

Metrics turn "this feels slow" into hard data. That lets you pinpoint bottlenecks, track gains over time, and benchmark against rivals.

"Load time" is vague — it lumps everything together. We break it into specific stages so you can analyze each part of the load.

First Contentful Paint (FCP) marks when the browser paints the first content — text or image. It tells users the page is alive, not blank.

FCP fires the moment the browser paints any HTML content, like a heading. It is the first visual feedback a user gets.

Time To Interactive (TTI) marks when the page is rendered and the main thread can reliably handle input — when users can actually click and type.

A fast paint is useless if clicks do nothing. Good TTI prevents "rage clicks" by ensuring the page responds when it looks ready.

Speed Index (SI) scores how quickly visible content fills in during load. Lower is better — it captures overall visual progress, not one moment.

Total Blocking Time (TBT) sums the time between FCP and TTI when the main thread was blocked. Long scripts block input — TBT quantifies that pain.

Beyond the basics, watch the Core Web Vitals: LCP (largest paint), CLS (layout shift), and FID (input delay) for a deeper UX view.

You've learned about several key performance metrics. Let's see if you can identify which one focuses on initial visual feedback.

You now know the core metrics: FCP, TTI, Speed Index, and TBT. Together they turn user experience into numbers you can act on.