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 | `

• 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)

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: 01clauding
• Source: 01clauding/claude-seo-skill
• License: MIT
• Homepage: https://clawpond.com/seo-skill

Install and usage instructions live in the source repository linked above.