---
name: shopify-theme-performance-d65b097bb09b
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)


---
Source attribution: jeremylongshore
Original project: https://github.com/jeremylongshore/claude-code-plugins-plus-skills/blob/ab85a061f7bcc039dac13ee193de7a4cd4388571/plugins/saas-packs/shopify-pack/skills/shopify-theme-performance/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Dave AI.

## Upstream license notice

MIT License

Copyright (c) 2025 Jeremy Longshore

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Dave AI resource metadata

Creator / publisher: jeremylongshore
Original project: https://github.com/jeremylongshore/claude-code-plugins-plus-skills/blob/ab85a061f7bcc039dac13ee193de7a4cd4388571/plugins/saas-packs/shopify-pack/skills/shopify-theme-performance/SKILL.md
Attribution basis: Source repository record

Fields of work: Design, Marketing
Roles: Content Marketer, Growth Marketer, Product Marketer, SEO Specialist, UX Designer
Specialties: API, Backend, Design, Frontend
Tools: Shopify
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: b2134b17436bbc131bf06c86f19c8aab9ecec14c435eca09f079856acb008212