Shopify Theme Performance

---
name: shopify-theme-performance
description: 'Optimize Shopify theme performance for Core Web Vitals (LCP, CLS, INP).

  Use when diagnosing slow page loads, fixing lazy-loaded hero images,

  profiling Liquid render times, or optimizing image delivery.

  Trigger with phrases like "shopify theme performance", "shopify core web vitals",

  "shopify lcp", "shopify liquid profiler", "shopify image optimization".

  '
allowed-tools: Read, Write, Edit, Bash(npm:*), Grep
version: 2.7.0
license: MIT
author: Jeremy Longshore <jeremy@intentsolutions.io>
tags:
- saas
- ecommerce
- shopify
compatibility: Designed for Claude Code
---
# Shopify Theme Performance

## Overview

59% of Shopify stores lazy-load their LCP image, adding 3-5 seconds to perceived load time. This skill covers the highest-impact theme optimizations: fixing LCP image loading, switching to the modern `image_url` filter, profiling Liquid render times, and optimizing font delivery.

## Prerequisites

- Access to the Shopify theme editor or theme files via CLI (`shopify theme dev`)
- Theme using Online Store 2.0 architecture (sections + JSON templates)
- Google Chrome DevTools or Lighthouse for measurement

## Instructions

### Step 1: Fix LCP Image Loading

The LCP element is usually the hero banner image. Find it in your theme's hero section:

```liquid
{% comment %} BAD: lazy-loading the LCP image costs 3-5s {% endcomment %}
{{ section.settings.hero_image | image_url: width: 1200 | image_tag: loading: 'lazy' }}

{% comment %} GOOD: eager load + fetchpriority for LCP {% endcomment %}
{{ section.settings.hero_image | image_url: width: 1200 | image_tag:
    loading: 'eager',
    fetchpriority: 'high',
    sizes: '100vw' }}
```

Add a preload hint in `theme.liquid` inside `<head>`:

```liquid
{%- if template.name == 'index' -%}
  <link rel="preload"
        href="{{ section.settings.hero_image | image_url: width: 1200 }}"
        as="image"
        fetchpriority="high">
{%- endif -%}
```

### Step 2: Use the `image_url` Filter

The modern `image_url` filter replaces the deprecated `img_url`. It supports responsive images via srcset:

```liquid
{% assign image = product.featured_image %}

<img src="{{ image | image_url: width: 800 }}"
     srcset="{{ image | image_url: width: 400 }} 400w,
             {{ image | image_url: width: 600 }} 600w,
             {{ image | image_url: width: 800 }} 800w,
             {{ image | image_url: width: 1200 }} 1200w"
     sizes="(max-width: 749px) 100vw, 50vw"
     width="{{ image.width }}"
     height="{{ image.height }}"
     loading="lazy"
     alt="{{ image.alt | escape }}">
```

Always set explicit `width` and `height` attributes to prevent CLS (Cumulative Layout Shift).

### Step 3: Liquid Profiler

Append `?profile=true` to any storefront URL to activate the Liquid profiler. It renders a table at the bottom of the page showing render times per snippet.

See [references/liquid-profiling.md](references/liquid-profiling.md) for how to read the output and common slow patterns.

### Step 4: Font Loading

Preload critical fonts and use `font-display: swap` to prevent invisible text:

```liquid
{% comment %} In theme.liquid <head> {% endcomment %}
<link rel="preload"
      href="{{ 'your-font.woff2' | asset_url }}"
      as="font"
      type="font/woff2"
      crossorigin>

<style>
  @font-face {
    font-family: 'YourFont';
    src: url('{{ "your-font.woff2" | asset_url }}') format('woff2');
    font-display: swap;
    font-weight: 400;
  }
</style>
```

Limit to 2-3 font files maximum. Each additional font file blocks rendering.

## Output

- LCP image loading eagerly with `fetchpriority="high"` and preload hint
- Responsive images using `image_url` with proper srcset and sizes
- Liquid profiler data identifying slow snippets
- Fonts preloaded with `font-display: swap`

## Error Handling

| Mistake | Impact | Fix |
|---------|--------|-----|
| Lazy-loading LCP image | +3-5s LCP | `loading="eager"` + `fetchpriority="high"` |
| Using `img_url` filter | No responsive images, larger payloads | Switch to `image_url` with width param |
| Render-blocking CSS | Delayed FCP | Inline critical CSS, defer non-critical |
| Too many Liquid `include` tags | Slow server render (variable scope leak) | Use `render` instead of `include` |
| Missing width/height on images | CLS jumps during load | Set explicit dimensions from `image.width`/`image.height` |
| Too many font files | Render blocking | Limit to 2-3 woff2 files, preload critical ones |

## Examples

### Fixing a Lazy-Loaded Hero Image

The homepage LCP score is 5+ seconds because the hero banner uses `loading="lazy"`. Switch to eager loading with fetchpriority and add a preload hint.

See [Core Web Vitals](references/core-web-vitals.md) for targets and measurement techniques.

### Migrating from img_url to image_url

Replace deprecated `img_url` filters with the modern `image_url` filter to enable responsive srcset images and reduce payload sizes.

See [Image Optimization](references/image-optimization.md) for the complete migration patterns.

### Profiling Slow Liquid Renders

A collection page takes 800ms to render server-side. Use the Liquid profiler to identify which snippets are slow and apply fixes.

See [Liquid Profiling](references/liquid-profiling.md) for profiler usage and common slow patterns.

## Resources

- [Shopify Theme Performance](https://shopify.dev/docs/storefronts/themes/best-practices/performance)
- [image_url Filter Reference](https://shopify.dev/docs/api/liquid/filters/image_url)
- Liquid Profiler
- [Core Web Vitals for Shopify](https://shopify.dev/docs/storefronts/themes/best-practices/performance#core-web-vitals)
- [font-display CSS Property](https://developer.mozilla.org/en-US/docs/Web/CSS/@font-face/font-display)