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Core Web Vitals (LCP/INP/CLS/FCP/TTFB) の閾値と改善のヒント

LCP

Largest Contentful Paint

Time until the largest visible content element (image, video, large text block) is rendered.

Thresholds

Good: ≤ 2.5 seconds

Needs improvement: ≤ 4 seconds

Poor: > 4 seconds

How to improve

Optimize hero images (modern formats, proper sizing, preload)
Reduce server response time (CDN, edge caching)
Eliminate render-blocking JS/CSS
Preload critical fonts; prefer system fonts for above-the-fold text

INP

Interaction to Next Paint

interactivity

Slowest-ish interaction latency observed during the page session (replaces FID since 2024).

Thresholds

Good: ≤ 200 ms

Needs improvement: ≤ 500 ms

Poor: > 500 ms

How to improve

Break up long tasks (>50ms) — yield to main thread
Defer non-critical JS (input handlers > rendering)
Avoid heavy work in event handlers
Use CSS transitions instead of JS animation where possible

CLS

Cumulative Layout Shift

stability

Sum of all unexpected layout shifts during the lifetime of the page (unitless score).

Thresholds

Good: ≤ 0.1 score

Needs improvement: ≤ 0.25 score

Poor: > 0.25 score

How to improve

Always set explicit width/height on images and ads
Reserve space for embeds before they load
Avoid inserting content above existing content (banners, cookie prompts)
Use transform: translate() instead of layout-affecting properties for animations

FCP

First Contentful Paint

Time until the first text or image is rendered (any DOM content).

Thresholds

Good: ≤ 1.8 seconds

Needs improvement: ≤ 3 seconds

Poor: > 3 seconds

How to improve

Reduce server response time
Eliminate render-blocking resources
Inline critical CSS

TTFB

Time to First Byte

Time between request and first byte of the response (server + network).

Thresholds

Good: ≤ 800 ms

Needs improvement: ≤ 1800 ms

Poor: > 1800 ms

How to improve

CDN + edge caching
Optimize backend (DB queries, N+1 etc.)
Use HTTP/2 or HTTP/3
Reduce redirect chains

使い方

各 Core Web Vital のカードを読みます: LCP、INP、CLS、FCP、TTFB。
各メトリクスの Color-coded バッジで表示される Good / Needs Improvement / Poor しきい値値に注目してください。
各メトリクックの下の箇条書きの改善ヒントを確認して、実行可能な修正を理解します。

例

LCP は Good ≤ 2500 ms、Needs Improvement ≤ 4000 ms、Poor > 4000 ms を表示しています。改善のヒントには、ヒーロー画像の最適化と render-blocking リソースの削除が含まれます。
CLS は Good ≤ 0.1、Poor > 0.25 を表示しています。ヒントには、広告と lazy-loaded 画像のスペース予約を取り上げて、レイアウトシフトを防ぐことが述べられています。

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