performance

A web-performance review guide for finding and fixing slow page loads, inefficient code, and heavy assets. It uses Lighthouse, a tool that audits website quality, and focuses on Core Web Vitals, Google's measures of loading, responsiveness, and visual stability.

In plain words
What is it for?
Use it to audit loading speed, reduce JavaScript, CSS, image, font, and third-party resource sizes, and improve server response and resource loading.
Why use it?
It helps identify which parts of a website make pages slow and decide what to fix first. It also gives performance limits and before-and-after measurements to check whether changes helped.

Skill for Claude CodeCodex

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 skills/dallay/agentsync/performance
Any agent
npx skills add dallay/agentsync --skill performance
Clone the repo
git clone --depth 1 https://github.com/dallay/agentsync

Made for: Claude Code, Codex.

Per session 50 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,402 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. Scan, not verified.
Origin 92% 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 $0.00050 $0.02402
Opus 5 $0.00025 $0.01201
Sonnet 5 $0.00010 $0.00480
Haiku 4.5 $0.00005 $0.00240

Measured 2d ago against content hash d934cf4c83fc, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

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.

return cached || fetch(event.request).then((response) => {
Origin

This is a copy

92% identical to performance — 134 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/skills/performance/SKILL.md · 388 lines

How it starts

The opening of the file, as written. The whole thing — 388 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Performance optimization

Deep performance optimization based on Lighthouse performance audits. Focuses on loading speed, runtime efficiency, and resource optimization.

How it works

  1. Identify performance bottlenecks in code and assets
  2. Prioritize by impact on Core Web Vitals
  3. Provide specific optimizations with code examples
  4. Measure improvement with before/after metrics

Performance budget

Resource Budget Rationale
Total page weight < 1.5 MB 3G loads in ~4s
JavaScript (compressed) < 300 KB Parsing + execution time
CSS (compressed) < 100 KB Render blocking
Images (above-fold) < 500 KB LCP impact
Fonts < 100 KB FOIT/FOUT prevention
Third-party < 200 KB Uncontrolled latency

Critical rendering path

Server response

  • TTFB < 800ms. Time to First Byte should be fast. Use CDN, caching, and efficient backends.
  • Enable compression. Gzip or Brotli for text assets. Brotli preferred (15-20% smaller).
  • HTTP/2 or HTTP/3. Multiplexing reduces connection overhead.
  • Edge caching. Cache HTML at CDN edge when possible.

Resource loading

Preconnect to required origins:

<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com" crossorigin>

Preload critical resources:

<!-- LCP image -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">

<!-- Critical font -->
<link rel="preload" href="/font.woff2" as="font" type="font/woff2" crossorigin>

Defer non-critical CSS:

<!-- Critical CSS inlined -->
<style>/* Above-fold styles */</style>

<!-- Non-critical CSS -->
<link rel="preload" href="/styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
<noscript><link rel="stylesheet" href="/styles.css"></noscript>

Read the full file on GitHub · 388 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 · 388 lines · 50 tokens per session scan A d934cf4c83fc

Subscribe to this mod's changes

performance is a skill published in the GitHub repository dallay/agentsync (54 stars, last pushed 2d ago), licensed MIT. It adds 50 tokens to every session and 2,402 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). It is 92% identical to performance, differing in 134 lines, and is treated as a copy.

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