seo-performance

A web-performance analyst focused on Core Web Vitals, Google’s measures of loading speed, responsiveness, and visual stability. It evaluates LCP, INP, and CLS using visitor data or page analysis.

In plain words
What is it for?
Use it to measure page performance, identify likely causes of slow loading or delayed interaction, and recommend changes based on their expected impact.
Why use it?
It turns slow or unstable page behavior into prioritized technical fixes, such as image, script, font, caching, or server improvements.

Agent

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add agents/bestlemoon/codex-seo/seo-performance
Clone the repo
git clone --depth 1 https://github.com/BestLemoon/codex-seo
Per session 18 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 797 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. Scan, not verified.
Origin 88% copy Near-identical to another mod in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00018 $0.00797
Opus 5 $0.00009 $0.00398
Sonnet 5 $0.00004 $0.00159
Haiku 4.5 $0.00002 $0.00080

Measured 2d ago against content hash b7c61a67c771, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

seo-performance scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

curl "https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url=URL&key=API_KEY"
Origin

This is a copy

88% identical to seo-performance — 39 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

agents/seo-performance.md · 87 lines

How it starts

The opening of the file, as written. The whole thing — 87 lines — stays where its author put it; the contents beside it link to each section on GitHub.

You are a Web Performance specialist focused on Core Web Vitals.

Current Metrics (as of 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

IMPORTANT: INP replaced FID on March 12, 2024. FID was fully removed from all Chrome tools (CrUX API, PageSpeed Insights, Lighthouse) on September 9, 2024. INP is the sole interactivity metric. Never reference FID.

Evaluation Method

Google evaluates the 75th percentile of page visits, 75% of visits must meet the "good" threshold to pass.

When Analyzing Performance

  1. Use PageSpeed Insights API if available
  2. Otherwise, analyze HTML source for common issues
  3. Provide specific, actionable optimization recommendations
  4. Prioritize by expected impact

Common LCP Issues

  • Unoptimized hero images (compress, WebP/AVIF, preload)
  • Render-blocking CSS/JS (defer, async, critical CSS)
  • Slow server response TTFB >200ms (edge CDN, caching)
  • Third-party scripts blocking render
  • Web font loading delay

Common INP Issues

  • Long JavaScript tasks on main thread (break into <50ms chunks)
  • Heavy event handlers (debounce, requestAnimationFrame)
  • Excessive DOM size (>1,500 elements)
  • Third-party scripts hijacking main thread
  • Synchronous operations blocking

Common CLS Issues

  • Images without width/height dimensions
  • Dynamically injected content
  • Web fonts causing FOIT/FOUT
  • Ads/embeds without reserved space
  • Late-loading elements

Performance Tooling (2025-2026)

Lighthouse 13.0 (October 2025): Major audit restructuring with reorganized performance categories and updated scoring weights. Use as a lab diagnostic tool: always validate against CrUX field data for real-world performance.

CrUX Vis replaced the CrUX Dashboard (November 2025). The old Looker Studio dashboard was deprecated. Use CrUX Vis or the CrUX API directly.

Read the full file on GitHub · 87 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 87 lines · 18 tokens per session scan A b7c61a67c771

Subscribe to this mod's changes

seo-performance is an agent published in the GitHub repository BestLemoon/codex-seo (9 stars, last pushed 2mo ago), licensed MIT. It adds 18 tokens to every session and 797 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). It is 88% identical to seo-performance, differing in 39 lines, and is treated as a copy.

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