shopify-theme-performance
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
Provided by Plugin
shopify-pack
Claude Code skill pack for Shopify (30 skills)
Installation
This skill is included in the shopify-pack plugin:
/plugin install shopify-pack@claude-code-plugins-plusClick to copy
Instructions
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:
{% 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 :
{%- 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 imageurl filter replaces the deprecated imgurl. It supports responsive images via srcset:
{% 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 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:
{% 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_urlwith 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 for targets and measurement techniques.
Migrating from imgurl to imageurl
Replace deprecated imgurl filters with the modern imageurl filter to enable responsive srcset images and reduce payload sizes.
See Image Optimization 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 for profiler usage and common slow patterns.