Performance Optimisation Reference

Performance Optimisation Reference

Expert guidance for optimising Shopify store performance including theme speed, asset optimisation, and Core Web Vitals.

Core Capabilities

1. Image Optimisation

Images are typically the largest assets - optimise aggressively.

Use Shopify CDN Image Sizing:

{# ❌ Don't load full-size images #}
<img src="{{ product.featured_image.src }}" alt="{{ product.title }}">

{# ✅ Use img_url filter with appropriate size #}
<img
  src="{{ product.featured_image | img_url: '800x800' }}"
  alt="{{ product.featured_image.alt | escape }}"
  loading="lazy"
  width="800"
  height="800"
>

Responsive Images:

<img
  src="{{ image | img_url: '800x' }}"
  srcset="
    {{ image | img_url: '400x' }} 400w,
    {{ image | img_url: '800x' }} 800w,
    {{ image | img_url: '1200x' }} 1200w,
    {{ image | img_url: '1600x' }} 1600w
  "
  sizes="(max-width: 600px) 400px, (max-width: 1200px) 800px, 1200px"
  alt="{{ image.alt | escape }}"
  loading="lazy"
  width="800"
  height="800"
>

Modern Image Formats:

<picture>
  {# WebP for modern browsers #}
  <source
    type="image/webp"
    srcset="
      {{ image | img_url: '400x', format: 'pjpg' }} 400w,
      {{ image | img_url: '800x', format: 'pjpg' }} 800w
    "
  >

  {# Fallback to JPEG #}
  <img
    src="{{ image | img_url: '800x' }}"
    srcset="
      {{ image | img_url: '400x' }} 400w,
      {{ image | img_url: '800x' }} 800w
    "
    alt="{{ image.alt | escape }}"
    loading="lazy"
  >
</picture>

Lazy Loading:

{# Native lazy loading #}
<img
  src="{{ image | img_url: '800x' }}"
  alt="{{ image.alt | escape }}"
  loading="lazy"
  decoding="async"
>

{# Eager load above-the-fold images #}
{% if forloop.index <= 3 %}
  <img src="{{ image | img_url: '800x' }}" loading="eager">
{% else %}
  <img src="{{ image | img_url: '800x' }}" loading="lazy">
{% endif %}

Preload Critical Images:

{# In <head> for hero images #}
<link
  rel="preload"
  as="image"
  href="{{ section.settings.hero_image | img_url: '1920x' }}"
  imagesrcset="
    {{ section.settings.hero_image | img_url: '800x' }} 800w,
    {{ section.settings.hero_image | img_url: '1920x' }} 1920w
  "
  imagesizes="100vw"
>

2. JavaScript Optimisation

Reduce JS payload and execution time.

Defer Non-Critical JavaScript:

{# ❌ Blocking JavaScript #}
<script src="{{ 'theme.js' | asset_url }}"></script>

{# ✅ Deferred JavaScript #}
<script src="{{ 'theme.js' | asset_url }}" defer></script>

{# ✅ Async for independent scripts #}
<script src="{{ 'analytics.js' | asset_url }}" async></script>

Inline Critical JavaScript:

{# Inline small, critical scripts #}
<script>
  // Critical initialisation code
  document.documentElement.classList.remove('no-js');
  document.documentElement.classList.add('js');
</script>

Code Splitting:

// Load features only when needed
async function loadCart() {
  const { Cart } = await import('./cart.js');
  return new Cart();
}

// Load on interaction
document.querySelector('.cart-icon').addEventListener('click', async () => {
  const cart = await loadCart();
  cart.open();
}, { once: true });

Remove Unused JavaScript:

// ❌ Don't load libraries you don't use
// Example: Don't include entire jQuery if you only need a few functions

// ✅ Use native alternatives
// Instead of: $('.selector').hide()
// Use: document.querySelector('.selector').style.display = 'none';

// Instead of: $.ajax()
// Use: fetch()

Minify JavaScript:

# Use build tools to minify
npm install terser --save-dev

# Minify
terser theme.js -o theme.min.js -c -m

3. CSS Optimisation

Optimise stylesheets for faster rendering.

Critical CSS:

{# Inline critical above-the-fold CSS in <head> #}
<style>
  /* Critical CSS only (header, hero) */
  .header { /* ... */ }
  .hero { /* ... */ }
  .button { /* ... */ }
</style>

{# Load full CSS deferred #}
<link
  rel="preload"
  href="{{ 'theme.css' | asset_url }}"
  as="style"
 
>
<noscript>
  <link rel="stylesheet" href="{{ 'theme.css' | asset_url }}">
</noscript>

Remove Unused CSS:

# Use PurgeCSS to remove unused styles
npm install @fullhuman/postcss-purgecss --save-dev

# Configure in postcss.config.js
module.exports = {
  plugins: [
    require('@fullhuman/postcss-purgecss')({
      content: ['./**/*.liquid'],
      defaultExtractor: content => content.match(/[\w-/:]+(?<!:)/g) || [],
    }),
  ],
};

Minify CSS:

# Use cssnano
npm install cssnano --save-dev

# Minify
npx cssnano style.css style.min.css

Avoid @import:

/* ❌ Don't use @import (blocks rendering) */
@import url('fonts.css');

/* ✅ Use multiple <link> tags instead */

<link rel="stylesheet" href="{{ 'main.css' | asset_url }}">
<link rel="stylesheet" href="{{ 'fonts.css' | asset_url }}">

4. Font Optimisation

Optimise web fonts for faster text rendering.

Font Loading:

{# Preload fonts #}
<link
  rel="preload"
  href="{{ 'font.woff2' | asset_url }}"
  as="font"
  type="font/woff2"
  crossorigin
>

{# Font face with font-display #}
<style>
  @font-face {
    font-family: 'CustomFont';
    src: url('{{ 'font.woff2' | asset_url }}') format('woff2');
    font-weight: 400;
    font-style: normal;
    font-display: swap; /* Show fallback font immediately */
  }
</style>

System Font Stack:

/* Use system fonts for instant rendering */
body {
  font-family:
    -apple-system,
    BlinkMacSystemFont,
    "Segoe UI",
    Roboto,
    "Helvetica Neue",
    Arial,
    sans-serif;
}

Subset Fonts:

/* Load only required characters */
@font-face {
  font-family: 'CustomFont';
  src: url('font-latin.woff2') format('woff2');
  unicode-range: U+0000-00FF, U+0131, U+0152-0153;
}

5. Liquid Template Optimisation

Optimise Liquid rendering for faster server response.

Cache Expensive Operations:

{# ❌ Repeated calculations #}
{% for i in (1..10) %}
  {{ collection.products.size }}  {# Calculated 10 times #}
{% endfor %}

{# ✅ Cache result #}
{% assign product_count = collection.products.size %}
{% for i in (1..10) %}
  {{ product_count }}
{% endfor %}

Use limit and offset:

{# ❌ Iterate full array and break #}
{% for product in collection.products %}
  {% if forloop.index > 5 %}{% break %}{% endif %}
  {{ product.title }}
{% endfor %}

{# ✅ Use limit #}
{% for product in collection.products limit: 5 %}
  {{ product.title }}
{% endfor %}

Avoid Nested Loops:

{# ❌ O(n²) complexity #}
{% for product in collection.products %}
  {% for variant in product.variants %}
    {# Expensive nested loop #}
  {% endfor %}
{% endfor %}

{# ✅ Flatten or preprocess #}
{% assign all_variants = collection.products | map: 'variants' | flatten %}
{% for variant in all_variants limit: 50 %}
  {{ variant.title }}
{% endfor %}

Prefer render over include:

{# ❌ include (slower, shared scope) #}
{% include 'product-card' %}

{# ✅ render (faster, isolated scope) #}
{% render 'product-card', product: product %}

Use section-specific stylesheets:

{# Scope CSS to section for better caching #}
{% stylesheet %}
  .my-section { /* ... */ }
{% endstylesheet %}

{# Scope JavaScript to section #}
{% javascript %}
  class MySection { /* ... */ }
{% endjavascript %}

6. Third-Party Script Optimisation

Minimise impact of external scripts.

Defer Third-Party Scripts:

{# ❌ Blocking third-party script #}
<script src="https://external.com/script.js"></script>

{# ✅ Async or defer #}
<script src="https://external.com/script.js" async></script>

{# ✅ Load on user interaction #}
<script>
  let gaLoaded = false;
  function loadGA() {
    if (gaLoaded) return;
    const script = document.createElement('script');
    script.src = 'https://www.googletagmanager.com/gtag/js?id=GA_ID';
    script.async = true;
    document.head.appendChild(script);
    gaLoaded = true;
  }

  // Load on scroll or after delay
  window.addEventListener('scroll', loadGA, { once: true });
  setTimeout(loadGA, 3000);
</script>

Use Facade Pattern:

{# Show placeholder instead of embedding heavy iframe #}
<div class="video-facade" data-video-id="abc123">
  <img src="thumbnail.jpg" alt="Video">
  <button Video</button>
</div>

<script>
  function loadVideo(btn) {
    const facade = btn.parentElement;
    const videoId = facade.dataset.videoId;
    const iframe = document.createElement('iframe');
    iframe.src = `https://www.youtube.com/embed/${videoId}?autoplay=1`;
    facade.replaceWith(iframe);
  }
</script>

7. Caching Strategies

Leverage browser and CDN caching.

Asset Versioning:

{# Shopify auto-versions assets #}
<link rel="stylesheet" href="{{ 'theme.css' | asset_url }}">
{# Outputs: /cdn/.../theme.css?v=12345678 #}

Long Cache Headers:

{# Shopify CDN sets appropriate cache headers #}
{# CSS/JS: 1 year #}
{# Images: 1 year #}

Service Worker (Advanced):

// sw.js - Cache static assets
self.addEventListener('install', event => {
  event.waitUntil(
    caches.open('v1').then(cache => {
      return cache.addAll([
        '/cdn/.../theme.css',
        '/cdn/.../theme.js',
        '/cdn/.../logo.png',
      ]);
    })
  );
});

self.addEventListener('fetch', event => {
  event.respondWith(
    caches.match(event.request).then(response => {
      return response || fetch(event.request);
    })
  );
});

8. Core Web Vitals Optimisation

Improve Google's Core Web Vitals metrics.

Largest Contentful Paint (LCP):

{# Optimise largest element load time #}

{# 1. Preload hero image #}
<link rel="preload" as="image" href="{{ hero_image | img_url: '1920x' }}">

{# 2. Use priority hint #}
<img src="{{ hero_image | img_url: '1920x' }}" fetchpriority="high">

{# 3. Optimise server response time (use Shopify CDN) #}

{# 4. Remove render-blocking resources #}
<script src="theme.js" defer></script>

Interaction to Next Paint (INP):

// 1. Reduce JavaScript execution time
// 2. Break up long tasks
function processItems(items) {
  // ❌ Long task
  items.forEach(item => processItem(item));

  // ✅ Break into smaller chunks
  async function processInChunks() {
    for (let i = 0; i < items.length; i++) {
      processItem(items[i]);

      // Yield to main thread every 50 items
      if (i % 50 === 0) {
        await new Promise(resolve => setTimeout(resolve, 0));
      }
    }
  }
  processInChunks();
}

// 3. Use requestIdleCallback
requestIdleCallback(() => {
  // Non-critical work
});

Cumulative Layout Shift (CLS):

{# 1. Always set width and height on images #}
<img
  src="{{ image | img_url: '800x' }}"
  width="800"
  height="600"
  alt="Product"
>

{# 2. Reserve space for dynamic content #}
<div style="min-height: 400px;">
  {# Content loads here #}
</div>

{# 3. Use aspect-ratio for responsive images #}
<style>
  .image-container {
    aspect-ratio: 16 / 9;
  }
</style>

9. Performance Monitoring

Track performance metrics.

Measure Core Web Vitals:

// Load web-vitals library
import { getCLS, getINP, getLCP } from 'web-vitals';

function sendToAnalytics({ name, value, id }) {
  // Send to analytics
  gtag('event', name, {
    event_category: 'Web Vitals',
    event_label: id,
    value: Math.round(name === 'CLS' ? value * 1000 : value),
  });
}

getCLS(sendToAnalytics);
getINP(sendToAnalytics);
getLCP(sendToAnalytics);

Performance Observer:

// Monitor long tasks
const observer = new PerformanceObserver(list => {
  for (const entry of list.getEntries()) {
    console.warn('Long task detected:', entry.duration, 'ms');
  }
});

observer.observe({ entryTypes: ['longtask'] });

Performance Checklist

Images:

• Use img_url filter with appropriate sizes
• Implement responsive images with srcset
• Add loading="lazy" to below-fold images
• Set explicit width and height attributes
• Preload critical hero images
• Use modern formats (WebP)

JavaScript:

• Defer or async all non-critical scripts
• Minify and bundle JavaScript
• Code-split large bundles
• Remove unused code
• Lazy load features on interaction

CSS:

• Inline critical CSS
• Defer non-critical CSS
• Remove unused styles
• Minify stylesheets
• Avoid @import

Fonts:

• Preload critical fonts
• Use font-display: swap
• Consider system font stack
• Subset fonts when possible

Third-Party:

• Audit all third-party scripts
• Load scripts async or on interaction
• Use facade pattern for heavy embeds
• Monitor third-party impact

Liquid:

• Cache expensive calculations
• Use limit instead of manual breaks
• Prefer render over include
• Avoid nested loops

Core Web Vitals:

• LCP < 2.5s
• INP < 200ms
• CLS < 0.1

Best Practices

1. Test on real devices - Mobile 3G performance matters
2. Use Lighthouse for performance audits
3. Monitor Core Web Vitals in production
4. Optimise above-the-fold content first
5. Lazy load everything else below the fold
6. Minimise main thread work for better interactivity
7. Use Shopify CDN for all assets
8. Version assets for effective caching
9. Compress images before uploading
10. Regular performance audits to catch regressions