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Core Web Vitals

최종 업데이트:

Definition

Core Web Vitals are the three core user experience metrics defined by Google.

Summary

Core Web Vitals consist of LCP (loading speed), INP (responsiveness), and CLS (visual stability). Google uses these metrics as Page Experience ranking signals, based on the 75th percentile (CrUX Field Data). Good thresholds are LCP ≤2.5s, INP ≤200ms, and CLS ≤0.1.

Three Metrics

LCP (Largest Contentful Paint) — Loading Speed

LCP measures the time until the largest content element (image or text block) in the viewport finishes rendering.

RatingLCP
Good2.5s or below
Needs Improvement2.5s – 4.0s
PoorAbove 4.0s

Main causes: slow server response (TTFB), render-blocking resources, poor image optimization. Improvements include CDN adoption, WebP/AVIF image conversion, and <link rel="preload">.

INP (Interaction to Next Paint) — Responsiveness

INP measures the time from user input to the next screen update. On March 12, 2024, it officially replaced FID (First Input Delay) in Core Web Vitals.

RatingINP
Good200ms or below
Needs Improvement200ms – 500ms
PoorAbove 500ms

Unlike FID, INP records the 75th percentile of all interactions during a page session. Clicks, taps, and keyboard input are all included.

CLS (Cumulative Layout Shift) — Visual Stability

CLS measures the cumulative score of unexpected layout movement during page load. Ads, images, and fonts loading and pushing content are typical examples.

RatingCLS
Good0.1 or below
Needs Improvement0.1 – 0.25
PoorAbove 0.25

Improvements: specify width and height on images and ads, set web fonts to font-display: optional, avoid placing dynamically inserted content at the top.

Measurement Methods

Field Data (Real User Data)

Google uses the 75th percentile standard. The worst value experienced by the top 25% of users must be within the Good threshold to receive a Good rating. Chrome User Experience Report (CrUX) is the data source, viewable in Google Search Console’s "Core Web Vitals" report.

Lab Data (Simulation Data)

Google PageSpeed Insights and Lighthouse measure metrics in a simulated environment. Lab Data can differ from Field Data; the official ranking criterion is Field Data.

SEO and Page Experience

Google began incorporating Core Web Vitals as Page Experience signals into search rankings from June 2021. Google Search Central states that "good page experience helps rankings, but content relevance is the most important factor." Poor-rated pages may rank lower under the same content quality conditions.

According to official web.dev data, improving Core Web Vitals has repeatedly been reported to improve business metrics such as bounce rate and conversion rate.

Relationship with AEO

Core Web Vitals are not direct AEO metrics, but when AI answer engines select citation sources based on the Google search index, they are indirectly connected. Google AI Overviews tends to prioritize top-ranking Google search pages as citation sources, and Core Web Vitals Poor ratings can lower search rankings and move sites away from AI citation candidates.

Korea Market Context

Korea has high mobile usage and strong LTE/5G infrastructure, so loading speed expectations are high. At the same time, ad-heavy news and commerce sites often have low CLS scores. Platform-based content such as Naver Blog and Tistory is hard to control directly for Core Web Vitals, but for self-operated sites, improving LCP and CLS is directly tied to search competitiveness.

Frequently Asked Questions

Q. Are Core Web Vitals and Lighthouse scores the same?
A. No. Lighthouse is Lab Data (simulation), while Core Web Vitals ranking criteria use CrUX Field Data (real users). A Lighthouse score of 100 can still show Poor in Field Data.

Q. INP is said to be harder to measure than FID — why?
A. FID measured only the first interaction, but INP tracks all clicks, taps, and key inputs throughout the session. INP varies with diverse user behavior patterns, making it hard to fully reproduce in lab environments.

Q. What if only some pages are Poor, not the whole site?
A. Google evaluates Core Web Vitals at the page level. Only Poor-rated pages are individually affected in rankings. Use the "By URL" filter in Google Search Console to identify problem pages.

Q. What work shows the fastest Core Web Vitals improvement?
A. Image optimization delivers the most immediate effect. WebP conversion plus loading="lazy" and width/height attributes improve both LCP and CLS with low implementation difficulty.

Q. What about new sites with no CrUX data?
A. Without sufficient traffic, there is no CrUX data and sites appear in the "Other" category. In that case, Page Experience signals do not apply. It is practical to build traffic with content first, then optimize Core Web Vitals.

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