Seo performance

seo-performance — Core Web Vitals & Page Performance

Core Web Vitals Thresholds (2026)

Metric	Good	Needs Improvement	Poor
LCP (Largest Contentful Paint)	≤2.5s	2.5s–4.0s	>4.0s
INP (Interaction to Next Paint)	≤200ms	200ms–500ms	>500ms
CLS (Cumulative Layout Shift)	≤0.1	0.1–0.25	>0.25

INP replaced FID on March 12, 2024. FID fully removed September 9, 2024. Never reference FID.

Key Statistics (2026)

• 77% of mobile pages now achieve "good" INP (up from 55% in 2022)
• 57.1% desktop / 49.7% mobile pass all three CWV (Oct 2025)
• December 2025 core update: Mobile CWV weighted more heavily in ranking calculations
• CWV evaluated at 75th percentile of field data — 75% of visits must meet "good"

HTTP/3 & QUIC Analysis

HTTP/3 reached 35% global adoption (2025), reducing mobile latency by 30%.

Check HTTP/3 Support

curl -sI --http3 https://example.com | grep -i "HTTP/3"
# Or check via headers:
curl -sI https://example.com | grep -i "alt-svc"

HTTP/3 Scoring

Status	Score	Recommendation
HTTP/3 enabled (Alt-Svc header present)	100	Excellent
HTTP/2 only	50	Enable HTTP/3 at CDN level (Cloudflare: default, AWS CloudFront: opt-in)
HTTP/1.1	0	Upgrade immediately — significant performance penalty

HTTP/3 is NOT a direct ranking factor but improves TTFB and LCP subparts measurably.

LCP Subpart Diagnostics (Feb 2025 CrUX Addition)

Break down LCP into 4 subparts for targeted optimization:

Subpart	What It Measures	Target
TTFB	Server response time	<800ms
Resource Load Delay	Time before LCP resource starts loading	Minimize (preload)
Resource Load Time	Time to download LCP resource	Compress, CDN, modern formats
Element Render Delay	Time from resource loaded to rendered	Reduce render-blocking CSS/JS

Performance Tooling (2025-2026)

• Lighthouse 13.0 (Oct 2025): Major audit restructuring, updated scoring weights
• CrUX Vis (Nov 2025): Replaced old Looker Studio CrUX Dashboard
• LCP subparts in CrUX (Feb 2025): TTFB, load delay, load time, render delay now available
• GSC 2025 (Dec 2025): AI-powered config suggestions, brand vs non-brand filters, hourly API data

Common Performance Issues

LCP Issues

• Unoptimized hero images → compress, WebP/AVIF, <link rel="preload">
• Render-blocking CSS/JS → defer, async, critical CSS inline
• Slow TTFB >800ms → edge CDN, server-side caching
• Web font loading delay → font-display: swap, preload

INP Issues

• Long JavaScript tasks >50ms → break into smaller chunks
• Heavy event handlers → debounce, requestAnimationFrame
• Excessive DOM >1,500 elements → virtualize lists
• Third-party scripts blocking main thread → async/defer

CLS Issues

• Images without width/height → always set dimensions or aspect-ratio
• Dynamically injected content → reserve space with min-height
• Web fonts FOIT/FOUT → font-display: optional or swap
• Ads/embeds without reserved space → explicit container sizing

Scoring

Category	Weight
LCP	35%
INP	30%
CLS	15%
HTTP/3	10%
Resource optimization	10%

PageSpeed Insights API v5

Quick Test

curl -s "https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url=https://example.com&strategy=mobile&category=performance&category=seo" \
  | jq '{performance: .lighthouseResult.categories.performance.score, seo: .lighthouseResult.categories.seo.score, lcp: .loadingExperience.metrics.LARGEST_CONTENTFUL_PAINT_MS, inp: .loadingExperience.metrics.INTERACTION_TO_NEXT_PAINT, cls: .loadingExperience.metrics.CUMULATIVE_LAYOUT_SHIFT_SCORE}'

Lab Data vs Field Data

Aspect	Lab (Lighthouse)	Field (CrUX)
Source	Simulated	Real Chrome users
Metrics	LCP, TBT, CLS, SI, FCP	LCP, INP, CLS, FCP, TTFB
Use case	Debugging, pre-launch	Ranking signal, trend analysis
Availability	Any URL	URLs with sufficient traffic only

Lighthouse CI for CI/CD

Set performance budgets in CI pipelines:

• LCP budget: 2500ms
• TBT budget: 200ms (lab proxy for INP)
• CLS budget: 0.1
• Fail build if budgets exceeded

CrUX API (Field Data)

Query Real User Metrics

POST https://chromeuxreport.googleapis.com/v1/records:queryRecord?key={API_KEY}
{
  "url": "https://example.com/page",
  "formFactor": "PHONE",
  "metrics": ["largest_contentful_paint", "interaction_to_next_paint", "cumulative_layout_shift"]
}

CrUX History API (2024)

• 25 weekly data points for trend analysis
• Detect performance regressions over time
• Correlate CWV changes with ranking changes

CrUX Vis (Nov 2025)

Replaced old Looker Studio CrUX Dashboard. Provides visual performance trends.

Reference Files

Load on-demand: references/cwv-thresholds.md for detailed thresholds, SPA handling, and measurement methodology.

Cross-reference: seo-data-tools skill for comprehensive GSC/GA4/PSI/CrUX integration guidance.

Output Format

Generate performance report with:

1. Performance Score (0-100)
2. CWV Status (pass/fail per metric with values)
3. HTTP/3 Status (enabled/disabled with Alt-Svc header)
4. LCP Subpart Breakdown (identify bottleneck subpart)
5. PSI Lab vs Field Comparison (if both available)
6. CrUX Trend (if history data available — improving/stable/degrading)
7. Prioritized Recommendations (by expected impact)