Performance Optimizer

Performance Optimizer Skill

Overview

This skill audits and fixes page load performance issues on websites. It analyzes images, CSS, JavaScript, fonts, and assets, then generates a before/after performance report with specific optimization recommendations and implementation steps.

Why Performance Matters

• User Experience: Pages that load in under 3 seconds have 40% higher conversion rates than those taking 6+ seconds
• SEO Rankings: Google uses Core Web Vitals as a ranking factor; slow sites rank lower
• Mobile Performance: 53% of users abandon sites that take over 3 seconds to load on mobile
• Cost Savings: Optimized sites consume less bandwidth and server resources
• Business Impact: Every 100ms of latency costs Amazon 1% in sales

Key Optimization Areas

1. Image Optimization

Why it matters: Images typically represent 50-80% of page weight. Optimization directly impacts load time.

Checks performed:

• Image dimensions: Ensures images aren't larger than displayed size
• Compression ratio: Recommends lossy compression for photos, lossless for graphics
• WebP conversion: Identifies images that should use modern WebP format (25-35% smaller)
• Responsive images: Detects missing srcset for different screen sizes
• Lazy loading: Identifies above-the-fold images loading eagerly vs below-the-fold images
• Format selection: PNG for graphics, JPEG for photos, SVG for icons, WebP for all

Deliverables:

• List of images requiring optimization
• Compression commands (ImageMagick, cwebp)
• HTML code samples for srcset implementation
• Lazy-loading implementation guidance

2. CSS Optimization

Why it matters: Render-blocking CSS prevents the page from painting. Unoptimized CSS increases file size and parse time.

Checks performed:

• Minification: Removes whitespace, comments, unnecessary characters
• Critical CSS: Identifies styles needed for above-the-fold content
• Unused CSS: Detects unused selectors (especially from frameworks like Bootstrap)
• Media query optimization: Ensures mobile-specific CSS isn't loaded on desktop
• CSS file splitting: Suggests code splitting for large stylesheets
• Animation efficiency: Recommends transform/opacity over layout-triggering properties

Deliverables:

• Minified CSS output
• Critical CSS extracted and ready for inlining
• PurgeCSS/Tailwind configuration for unused CSS removal
• CSS splitting strategy guide

3. JavaScript Optimization

Why it matters: JavaScript is the most expensive resource. Parse, compile, and execute time directly impacts performance.

Checks performed:

• Minification: Removes whitespace and renames variables
• Code splitting: Identifies opportunities to split bundles for different routes
• Defer/async loading: Recommends delaying non-critical scripts
• Bundle analysis: Shows which libraries consume the most bytes
• Tree-shaking: Identifies unused exports that can be removed
• Polyfill optimization: Removes polyfills needed for older browsers you don't support

Deliverables:

• Minified JavaScript output
• Code splitting recommendations with Vite configuration
• Bundle analysis report with size breakdown
• Script loading strategy (defer, async, preload)

4. Font Optimization

Why it matters: Web fonts block text rendering. Poor font strategy causes invisible text for 3-10 seconds.

Checks performed:

• Font subsetting: Identifies opportunity to include only used characters
• Font-display strategy: Recommends swap for system fonts, fallback for custom fonts
• Preloading: Ensures critical fonts load before content needs them
• Variable fonts: Suggests replacing multiple font files with single variable font
• Redundant fonts: Detects duplicate font families from multiple sources

Deliverables:

• Font-display CSS with recommended values
• Preload link tags
• Subsetting commands for Google Fonts or custom fonts
• Variable font migration guide

5. Asset Preloading and Prefetching

Why it matters: Strategic preloading/prefetching reduces perceived load time and improves interaction metrics.

Checks performed:

• Critical resources: Identifies files needed for LCP (Largest Contentful Paint)
• DNS prefetch: Detects slow third-party domains
• Preconnect: Identifies resource origin connections to establish early
• Prefetch: Identifies likely next page resources to load opportunistically

Deliverables:

• Preload/prefetch link tags with priorities
• Third-party script optimization strategy
• DNS prefetch recommendations

6. Lighthouse Score Targeting

Why it matters: Lighthouse is the industry standard. Scores below 90 indicate room for improvement.

Target scores:

• Performance: 90+
• Accessibility: 90+
• Best Practices: 90+
• SEO: 90+

Metrics analyzed:

• First Contentful Paint (FCP): Time until first content paints
• Largest Contentful Paint (LCP): Time until largest visual element paints
• Cumulative Layout Shift (CLS): Unexpected layout movement
• Time to Interactive (TTI): Time until page is fully interactive
• Total Blocking Time (TBT): Time JavaScript blocks main thread

How to Use This Skill

Step 1: Provide Site Information

Give me:

• Site URL (or local directory path)
• Site technology stack (React, Vue, static HTML, etc.)
• Target browsers/devices
• Current performance concerns (if known)

Step 2: Audit Review

I will:

• Run Lighthouse audit on your site
• Analyze all assets (images, CSS, JS, fonts)
• Identify performance bottlenecks
• Prioritize fixes by impact

Step 3: Optimization Plan

I provide:

• Before/after screenshot comparison
• Specific recommendations with estimated impact
• Implementation steps for each optimization
• Code examples ready to use

Step 4: Implementation Support

I can:

• Write optimization code (minified CSS/JS)
• Generate srcset and lazy-loading HTML
• Create Lighthouse configuration for specific scenarios
• Verify improvements with follow-up audits

Optimization Priorities

High Impact (Do First)

1. Image optimization - 30-50% byte reduction typical
2. JavaScript minification and code splitting - 20-40% reduction
3. Unused CSS removal - 20-60% reduction
4. Font subsetting/display optimization - 50-80% faster text rendering

Medium Impact

1. Critical CSS inlining - Eliminates render-blocking CSS
2. Script defer/async - Improves Time to Interactive
3. Asset preloading - 10-20% LCP improvement
4. Compression (gzip/brotli) - 30-50% byte reduction

Low Impact (Nice to Have)

1. Service Worker caching
2. CDN optimization
3. HTTP/2 Push (mostly superseded by preload)
4. WebAssembly migration (for specific use cases)

Output Deliverables

Performance Report

• Lighthouse scores (before/after)
• Byte savings per optimization
• Load time improvements (estimated)
• Priority recommendations
• Implementation timeline

Implementation Files

• Optimized images with WebP alternatives
• Minified CSS/JS
• HTML snippets for srcset, lazy-loading, preload
• Build configuration updates (webpack, Vite, etc.)
• Performance budget recommendations

Verification Plan

• Steps to verify each optimization
• How to measure real-world performance
• Continuous monitoring recommendations

Common Questions

Q: Will optimizations affect functionality? A: No. Performance optimizations maintain full functionality while reducing file size and load time.

Q: Can I apply optimizations gradually? A: Yes. The report prioritizes by impact, so you can implement high-impact optimizations first.

Q: Do I need expensive tools? A: No. All recommendations use free tools (ImageMagick, Lighthouse CLI, PurgeCSS, etc.).

Q: How often should I re-audit? A: After major changes. At minimum, quarterly to catch performance regressions.

Tools Used

• Lighthouse CLI (audits performance)
• ImageMagick (image optimization)
• cwebp (WebP conversion)
• PurgeCSS / Tailwind (unused CSS removal)
• Source maps (bundle analysis)
• Google Fonts API (font optimization)

Source & license

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

• Author: mrhinkle
• Source: mrhinkle/VibeCoding-a-Website
• License: MIT

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