Perf

Audit performance — bundle size, image weight, render-blocking resources, and Core Web Vitals via PageSpeedFrom its SKILL.md

Install

npx -y skills add Surfrrosa/claude-skills --skill perf

Assembled from the repository path, not quoted from the project. Check it against their README if it does not work.

3 things to look at

1 stars1 stars. Stars are a popularity signal and not a quality one, but at this level it is likely that nobody has read this closely except its author, and you would be relying on your own review.
runs commandsInstructs the agent to run 1 command, including `next build`.
fetches URLsInstructs the agent to fetch 2 URLs, including https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url={URL}&category=performance&strategy=mobile and 1 more.

SKILL.md

13.8 KB, ~3.6k tokens by cl100k_base, as published. Nobody here has run it

Performance Audit

Analyze a project for performance problems that kill load times and conversions. Two modes: source analysis (what you can catch from code) and live analysis (what Google PageSpeed Insights sees).

Sample output

A successful run produces something like:

## Performance Audit — marketing-site
Stack: Next.js 16

### Images (3.4MB total)
- public/hero.png — 1.8MB PNG, should be WebP (~250KB)
- public/team-photo.jpg — 920KB, unresponsive (no srcset)
- 6 images below the fold loading eagerly (missing loading="lazy")

### JavaScript (480KB bundled)
- src/components/Analytics.tsx — loads PostHog synchronously in <head>; should be async or deferred
- 3 client components could be server components (no hooks, no event handlers)

### Fonts
- Google Fonts <link> missing &display=swap — invisible text during font load

### Caching & Compression
- public/_headers missing — no long cache on static assets

### Summary
- Critical: 2 (hero image weight, synchronous analytics)
- Warning: 3
- Top priority: convert hero.png to WebP (saves ~1.5MB on first load)

Arguments

The user may specify:

No args: audit current working directory (source only)
A project name or path (e.g., myapp, marketing-site)
--live: also hit PageSpeed Insights API for Core Web Vitals on the live URL
--fix: auto-fix safe categories
all --live: audit all web projects against their live URLs

Parse the argument string. Examples:

/perf -- source audit on current project
/perf myapp --live -- source + PageSpeed for myapp
/perf all --live -- PageSpeed for all sites (skip source, just compare live scores)
/perf myapp --fix -- source audit + auto-fix for myapp

Project Registry

Alias	Path	Live URL	Stack
`myapp`	(add your projects here)	(add your projects here)	(add your projects here)

Source Audit

Step 0: Detect the project

Resolve path from alias or cwd
Detect stack: Next.js, Astro, Flask/Jinja, static HTML
Read CLAUDE.md and build config (next.config.*, astro.config.*, vite.config.*) for relevant settings

Step 1: Image Weight

Images are the #1 cause of slow pages for most sites. Check thoroughly.

1a. Image formats

Search public/, static/, src/assets/, src/images/, and any other asset directories for image files.

Flag unoptimized formats: .png and .jpg/.jpeg files that should likely be .webp or .avif. Exceptions:
- Favicons (favicon.ico, favicon.png) -- these need broad format support
- OG/social images -- some platforms don't support WebP in meta tags yet. Check on a case-by-case basis.
- SVGs are fine as-is
Flag oversized files: Any image over 200KB. Any image over 500KB is critical.
Count total image weight: Sum all image file sizes in the project. Compare to a budget:
- Under 1MB total: good
- 1-3MB: warning
- Over 3MB: critical

1b. Image optimization pipeline

Check whether the project has an image optimization strategy:

Stack	What to check
Next.js	Is `next/image` or `<Image>` used? Check for raw `<img>` tags in components. `next/image` auto-optimizes.
Astro	Is `astro:assets` or `<Image>` from `astro:assets` used? Check for raw `<img>` tags.
Flask/Jinja	No built-in optimization. Check for a build step (sharp, imagemin, squoosh) or CDN with transforms.
Static HTML	Check for raw `<img>` tags pointing to unoptimized files.

Flag: Raw <img> tags pointing to large files when the framework provides an optimized alternative.

1c. Responsive images

Check <img> tags for srcset and sizes attributes (or framework equivalents that generate them).
Check for images with fixed width/height in pixels that are larger than needed on mobile.
Above-the-fold hero images should have loading="eager" or fetchpriority="high". All other images should have loading="lazy".

Flag: Large images without responsive variants, no lazy loading on below-fold images, no priority hints on hero images.

Step 2: JavaScript Weight

2a. Bundle analysis

For projects with a build step:

Stack	How to check
Next.js	Look for `next build` output in `.next/` or run the build and check the output summary. Check for `@next/bundle-analyzer` in dependencies.
Astro	Astro ships zero JS by default. Check for `client:*` directives -- each one adds a JS island. Count them.
Vite	Check `dist/assets/` for JS file sizes after build.
Static HTML	Count and size all `<script>` tags.

Flag: Any single JS bundle over 200KB (gzipped). Any total JS payload over 500KB.
Flag: Third-party scripts loaded synchronously in <head> without async or defer.

2b. Third-party scripts

Search for external script URLs in templates/layouts:

Analytics scripts (PostHog, Google Analytics, Plausible, etc.)
Chat widgets, support tools
Font loaders
Ad scripts
Social embeds

For each, check:

Is it loaded with async or defer?
Is it loaded from a CDN or self-hosted?
Could it be loaded later (after interaction, after page load)?

Flag: Synchronous third-party scripts in <head>, more than 3 third-party scripts total, any third-party script without async/defer.

2c. Unused JavaScript

For Next.js: check for use client components that could be server components. Any component that doesn't use hooks, event handlers, or browser APIs doesn't need use client.
For Astro: check for client:load directives that could be client:idle or client:visible (defer hydration until needed).
Search for large libraries imported but potentially underused: moment (use date-fns), lodash (use individual imports), full @mui/material or @chakra-ui/react imports.

Flag: Client components that could be server components, eager hydration that could be deferred, heavy libraries with lighter alternatives.

Step 3: CSS Weight

3a. CSS delivery

Check for large CSS files loaded in <head> (render-blocking).
Check for inline critical CSS in the <head> with deferred loading of the full stylesheet.
For Tailwind projects: check if CSS purging is configured (it is by default in v3+, but verify in tailwind.config.*).

Flag: CSS files over 50KB in <head> without critical CSS extraction, Tailwind without purge.

3b. Unused CSS

For Tailwind: this is handled by purge. Just verify it's on.
For vanilla CSS: check if the project has multiple stylesheets with overlapping selectors or dead selectors (hard to check exhaustively from source, but flag obviously large CSS files).

Step 4: Font Loading

Search for @font-face declarations, Google Fonts links, or font files in assets:

Flag: Google Fonts loaded via <link> in <head> without display=swap parameter (blocks rendering while fonts load).
Flag: More than 3 font files/variants loaded (each is a separate network request).
Flag: Font files over 100KB each (should be subset or use woff2).
Check: Is font-display: swap set in @font-face declarations? Missing = invisible text during load.
Good: Self-hosted woff2 fonts with font-display: swap and preload hints.

Step 5: Caching and Compression

Check deploy/server config for:

Cache headers: Check vercel.json, _headers (Netlify), railway.json, server code, or .htaccess for cache-control headers on static assets. Images, fonts, and hashed JS/CSS should have long cache (max-age=31536000, immutable). HTML should have short/no cache.
Compression: Most CDNs (Vercel, Netlify, Cloudflare) handle gzip/brotli automatically. For self-hosted (Railway/Flask): check for compression middleware (flask-compress, gzip middleware, nginx config).

Flag: No compression for self-hosted projects, no cache headers for static assets.

Step 6: Stack-Specific Checks

Next.js

next/image usage for all content images
next/font for font loading (auto-optimizes and self-hosts)
next/script for third-party scripts with strategy prop
generateStaticParams for static generation of dynamic routes
ISR/SSG vs SSR: are pages that could be static being server-rendered on every request?
Check for dynamic = 'force-dynamic' or revalidate = 0 on pages that don't need it

Astro

Minimal client directives (zero JS is the goal)
<Image> component for auto-optimization
Static output mode when possible
Check astro.config.mjs for compressHTML: true

Flask/Jinja

Static files served via CDN or with cache headers?
flask-compress or equivalent for gzip?
Template caching enabled in production?
Are database queries in views optimized? (N+1 queries, missing indexes -- hard to check from source but flag obvious patterns like queries in loops)

Static HTML

Files minified? (HTML, CSS, JS)
Are assets fingerprinted/hashed for cache busting?
CDN in front of hosting?

Live Audit (--live flag)

Use the Google PageSpeed Insights API via WebFetch:

https://www.googleapis.com/pagespeedonline/v5/runPagespeed?url={URL}&category=performance&strategy=mobile

Also run with strategy=desktop for comparison.

Parse the response for:

Core Web Vitals

Metric	Good	Needs Work	Poor
LCP (Largest Contentful Paint)	< 2.5s	2.5-4s	> 4s
INP (Interaction to Next Paint)	< 200ms	200-500ms	> 500ms
CLS (Cumulative Layout Shift)	< 0.1	0.1-0.25	> 0.25

Additional Metrics

FCP (First Contentful Paint)
Speed Index
Total Blocking Time (TBT)
Time to Interactive (TTI)

Opportunities

Parse the lighthouseResult.audits for failed audits and their savings estimates. The most common ones:

render-blocking-resources -- CSS/JS blocking first paint
unused-javascript -- JS loaded but not executed
unused-css-rules -- CSS loaded but not applied
modern-image-formats -- images that should be WebP/AVIF
efficiently-encode-images -- images that could be compressed further
properly-size-images -- images larger than their display size
offscreen-images -- below-fold images loaded eagerly

Output Format

Single Project (source)

## Performance Audit -- [project name]
**Stack:** [detected stack]

### Images ([total weight])
- [findings]

### JavaScript ([total weight])
- [findings]

### CSS
- [findings]

### Fonts
- [findings]

### Caching & Compression
- [findings]

### Stack-Specific
- [findings]

### Summary
- **Critical:** [count] -- [list]
- **Warning:** [count] -- [list]
- **Top priority:** [the single most impactful fix]

Single Project (source + live)

Same as above, plus:

### PageSpeed Scores
| | Mobile | Desktop |
|---|--------|---------|
| Performance | [score] | [score] |
| LCP | [value] | [value] |
| INP | [value] | [value] |
| CLS | [value] | [value] |
| FCP | [value] | [value] |
| TBT | [value] | [value] |

### Top Opportunities (from Lighthouse)
1. [opportunity] -- est. [savings]
2. [opportunity] -- est. [savings]
3. [opportunity] -- est. [savings]

All Projects (--live only)

## Performance Audit -- All Sites

| Site | Mobile | Desktop | LCP | CLS | INP | Top Issue |
|------|--------|---------|-----|-----|-----|-----------|
| myapp | 82 | 95 | 2.1s | 0.02 | 120ms | -- |
| myapp | 94 | 99 | 1.2s | 0.01 | 80ms | -- |
| ... | ... | ... | ... | ... | ... | ... |

Sort by mobile score ascending (worst first).

Fix Mode (--fix flag)

Safe to auto-fix:

Add loading="lazy" to below-fold <img> tags that are missing it
Add fetchpriority="high" to above-fold hero images
Add decoding="async" to images
Add async or defer to synchronous <script> tags in <head> (unless they must be synchronous)
Add font-display: swap to @font-face declarations missing it
Add &display=swap to Google Fonts <link> URLs missing it
Replace <img> with framework <Image> component where applicable and straightforward
Add compressHTML: true to Astro config if missing

Requires confirmation:

Replacing image formats (PNG/JPG to WebP) -- needs build pipeline
Adding next/font or self-hosted fonts -- changes font loading behavior
Changing client:load to client:visible in Astro -- may break user expectations
Converting SSR routes to SSG -- changes data freshness model

Never auto-fix:

Bundle splitting strategy
Third-party script removal
Image content resizing (need to know target dimensions)
Caching policy changes (affects freshness)
Database query optimization

Rules

Images first. For most sites, image optimization gives the biggest performance win for the least effort. Always lead with image findings.
Mobile matters more. PageSpeed mobile scores are what Google uses for ranking. Always show mobile prominently.
Don't shame slow scores. A 60 on mobile for a dynamic Flask app with no CDN is expected. Context matters. Focus on what's fixable.
Be specific about savings. "Optimize images" is useless. "Convert hero.png (1.2MB) to WebP (~180KB) -- saves ~1MB on first load" is actionable.
Don't duplicate other skills. This is not /seo (no meta tags), not /a11y (no accessibility), not /ship (no deployment checks). Strictly performance.
PageSpeed API is free but rate-limited. Don't hammer it. One request per URL per strategy is enough. If running all, add a brief pause between requests.
Source analysis is always valuable. Even without --live, you can catch most issues from the code. Live mode adds real numbers but the source audit is where fixes come from.

Skill Run Logging

After completing this skill, append a log entry to ~/.claude/skill-log.md under the Run Log table. New entries at the top (most recent first).

What ships with it

Read from the repository

Just SKILL.md. No reference files, no scripts.