What metrics are used to measure site speed?

Several key metrics are used to evaluate the speed and "responsiveness" of a site. They show how fast a page loads, when the user sees content, and how soon they can start interacting with the site.

Below are the most important and most commonly used ones.

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#Core Web Vitals (Google's core metrics)

This is the main set of metrics that both search engines and developers rely on.

#LCP (Largest Contentful Paint)

Shows when the largest visible element on screen appeared (an image, a banner, a heading).

• Good: under 2.5 sec
• Bad: the user sees a blank screen or a skeleton for a long time

Put simply: "How quickly the page appears loaded"

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#INP (Interaction to Next Paint)

Measures how quickly the site responds to user actions (a click, typing text).

• Good: under 200 ms
• Bad: buttons respond with a delay

"How much the site 'lags' during interaction"

(FID used to be used instead of INP, but it's now considered deprecated.)

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#CLS (Cumulative Layout Shift)

Shows how much content jumps around during loading.

• Good: under 0.1
• Bad: buttons and text shift, and the user clicks the wrong thing

"How stable the page's layout is"

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#Page load metrics

These help you understand what exactly happens during loading.

#TTFB (Time To First Byte)

The time from the request to receiving the first byte from the server.

• Affected by the server, caching, SSR
• A good value is under 500 ms

"How quickly the server responds"

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#FCP (First Contentful Paint)

Shows when the first content appears on screen (text, an icon, a background).

"When the user realizes the site has started loading"

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#TTI (Time To Interactive)

The time when the page is not only visible, but also fully ready for interaction.

"When the site can actually be used without lag"

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#Why these metrics matter

• They affect the user's perception of speed
• They're tied to SEO and ranking
• They help you understand exactly where the bottleneck is: the server, JS, styles, images

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#In short

• LCP - how fast the main content appears
• INP - how quickly the site responds
• CLS - whether the interface jumps around
• TTFB - the server's response speed
• FCP / TTI - the sense of loading starting and the page being ready