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webperf-loading

webperf-loading is a skill for Claude Code from nucliweb/webperf-snippets. It costs 172 tokens per session (3,866 once invoked), scanned A, original, MIT.

A set of Chrome DevTools scripts for checking how quickly a web page loads and what delays it. It examines server response time, blocking files, scripts, fonts, images, and browser loading behavior.

In plain words
What is it for?
Use it to investigate slow server responses, render-blocking CSS or JavaScript, third-party scripts, font loading, image loading, caching, redirects, and browser performance measurements.
Why use it?
It helps locate the specific files or loading steps that make a page appear late or respond slowly, instead of relying on guesswork.

Skill for Claude Code

Written for Claude Code: installed under .claude/.

not rated 1.5krepo +1 changed 3d ago A scan SkillSpector: warn 172 tokens original MIT

Good fit Use it to investigate slow server responses, render-blocking CSS or JavaScript, third-party scripts, font loading, image loading, caching, redirects, and browser performance measurements.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/nucliweb/webperf-snippets/webperf-loading
View source ↗ nucliweb/webperf-snippets webperf-snippets.net
About the project

WebPerf Snippets is a collection of JavaScript snippets that run in browser developer tools to measure and debug website performance. It helps developers investigate Core Web Vitals, loading, interaction responsiveness, images, video, scripts, and other performance issues. Its catalogue skills, setting, and instruction support agent-assisted use of the snippets.

nucliweb/webperf-snippets · 1,454 stars · on GitHub · webperf-snippets.net

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.

Any agent
npx skills add nucliweb/webperf-snippets --skill webperf-loading
Clone the repo
git clone --depth 1 https://github.com/nucliweb/webperf-snippets

Made for: Claude Code.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for webperf-loading

README.md
[![agentmods](https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-loading/github.svg)](https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-loading)
Your own site
<a href="https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-loading"><img src="https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-loading/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for webperf-loading

Your own site · 80×15
<a href="https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-loading"><img src="https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-loading.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 172 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,866 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits
  • NVIDIA SkillSpector warn 7 Sept 2026
SkillSpector: 1 finding, up to medium

These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →

  • medium Excessive Agency · line 122
    Skill enables autonomous high-impact decisions without human-in-the-loop verification. Critical operations (destructive commands, financial transactions, data deletion) should require explicit user confirmation.
    Fix: Add human-in-the-loop confirmation for destructive, irreversible, or high-impact operations. Never auto-execute commands that modify files, send data, or alter system state.
How audits are shown
Origin original No closer match found 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 5.1 $0.00172 $0.03866
Opus 5.5 $0.00069 $0.01546
Sonnet 5.5 $0.00034 $0.00773
Haiku 4.5 $0.00017 $0.00387

Measured 3d ago against content hash fac25d3d6fd2, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

webperf-loading scanned grade A with 0 findings 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 3d ago.

The scan reads SKILL.md. This mod also ships 34 executable files (scripts/Back-Forward-Cache.js, scripts/Cache-Strategy-Analysis.js, scripts/Client-Side-Redirect-Detection.js, …), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

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.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

.claude/skills/webperf-loading/SKILL.md · 276 lines

How it starts

The opening of the file, as written. The whole thing — 276 lines — stays where its author put it; the contents beside it link to each section on GitHub.

WebPerf: Loading Performance

JavaScript snippets for measuring web performance in Chrome DevTools.

Running a script

  1. Run the script with mcp__chrome-devtools__evaluate_script and read the object it returns. The scripts have no console output, so the returned object is the result: script, status and the metric data (value, rating, details, items, issues), as described in references/schema.md.
  2. When status is "tracking", the script keeps observing the page. After the interactions, call the function named in getDataFn with another mcp__chrome-devtools__evaluate_script call, for example getINP(), and read the object it returns.
  3. When status is "error" or "unsupported", read error: the page may not have loaded yet, or the browser lacks the API.

Without Chrome DevTools MCP, run the same snippet with the webperf-snippets CLI, which loads the page in headless Chromium and prints the same result objects: npx webperf-snippets <url> --snippet <Name> --json, where <Name> is the script name without .js. It needs Playwright and Chromium (npm install --save-dev webperf-snippets playwright, then npx playwright install chromium), and tracking snippets take --interact-script <file> to drive the interactions.

Scripts

  • scripts/Back-Forward-Cache.js — Back Forward Cache
  • scripts/CSS-Media-Queries-Analysis.js — CSS media queries analysis
  • scripts/Cache-Strategy-Analysis.js — Cache Strategy Analysis
  • scripts/Client-Side-Redirect-Detection.js — Client Side Redirect Detection
  • scripts/Compression-Audit.js — Compression audit
  • scripts/Content-Visibility.js — Content visibility
  • scripts/Critical-CSS-Detection.js — Critical CSS detection
  • scripts/Event-Processing-Time.js — Event processing time
  • scripts/FCP.js — First contentful paint (FCP)
  • scripts/Find-Above-The-Fold-Lazy-Loaded-Images.js — Find above the fold lazy loaded images
  • scripts/Find-Images-With-Lazy-and-Fetchpriority.js — Find images with loading lazy and fetchpriority
  • scripts/Find-non-Lazy-Loaded-Images-outside-of-the-viewport.js — Find non lazy loaded images outside of the viewport
  • scripts/Find-render-blocking-resources.js — Find render-blocking resources
  • scripts/First-And-Third-Party-Script-Info.js — First And Third Party Script Info
  • scripts/First-And-Third-Party-Script-Timings.js — First and third party script timings
  • scripts/Fonts-Preloaded-Loaded-and-used-above-the-fold.js — Fonts preloaded, loaded, and used above the fold
  • scripts/Inline-CSS-Info-and-Size.js — Inline CSS info and size
  • scripts/Inline-Script-Info-and-Size.js — Inline script info and size
  • scripts/JS-Execution-Time-Breakdown.js — JavaScript execution time breakdown
  • scripts/Prefetch-Resource-Validation.js — Prefetch resource validation
  • scripts/Priority-Hints-Audit.js — Priority hints audit
  • scripts/Resource-Hints-Validation.js — Resource Hints Validation
  • scripts/Resource-Hints.js — Resource hints
  • scripts/SSR-Hydration-Data-Analysis.js — SSR framework hydration data analysis
  • scripts/Script-Loading.js — Scripts loading
  • scripts/Server-Timing-Early-Hints.js — Server-Timing and Early Hints viewer
  • scripts/Service-Worker-Analysis.js — Service worker analysis
  • scripts/Speculation-Rules-Inspector.js — Speculation rules inspector
  • scripts/TTFB-Resources.js — Time to first byte: Measure TTFB for all resources
  • scripts/TTFB-Sub-Parts.js — Time to first byte: Measure TTFB sub-parts
  • scripts/TTFB.js — Time to first byte: Measure the time to first byte
  • scripts/Third-Party-Impact-by-Domain.js — Third-party impact by domain
  • scripts/Validate-Preload-Async-Defer-Scripts.js — Validate Preload Async Defer Scripts
  • scripts/Webfont-Usage-Analyzer.js — Webfont usage analyzer

Read the full file on GitHub · 276 lines

Files

What ships with it

36 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

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. 3d ago Changed · -1 lines fac25d3d6fd2
  2. 6d ago Changed · +8 lines 1d8e2a67e8ff
  3. 7d ago Changed d32afcf0585a
  4. 22d ago Changed · +51 lines 4742c0787328
  5. 23d ago Changed f143cb42c20e
  6. 1mo ago First seen · 218 lines · 172 tokens per session scan A 657830449fb5

Subscribe to this mod's changes

webperf-loading is a skill published in the GitHub repository nucliweb/webperf-snippets (1,454 stars, last pushed today), licensed MIT. It adds 172 tokens to every session and 3,866 once invoked, about $0.0007 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

Analytics and metricsFrontendperformanceperformance-analysisperformance-metricssnippetswebperfwebperformance
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