Imported from jeremylongshore/tons-of-skills-marketplace (
skills/.curated/shopify-theme-performance/SKILL.md). Install upstream withnpx skills add jeremylongshore/tons-of-skills-marketplace --skill shopify-theme-performance. Copyright stays with the author (MIT).
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 <head>:
{%- 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:
{% 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 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 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.