From shopify-pack
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".
npx claudepluginhub flight505/skill-forge --plugin shopify-packThis skill is limited to using the following tools:
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.
Guides Next.js Cache Components and Partial Prerendering (PPR): 'use cache' directives, cacheLife(), cacheTag(), revalidateTag() for caching, invalidation, static/dynamic optimization. Auto-activates on cacheComponents: true.
Guides building MCP servers enabling LLMs to interact with external services via tools. Covers best practices, TypeScript/Node (MCP SDK), Python (FastMCP).
Share bugs, ideas, or general feedback.
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.
shopify theme dev)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 -%}
image_url FilterThe 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).
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.
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.
fetchpriority="high" and preload hintimage_url with proper srcset and sizesfont-display: swap| 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 |
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.
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.
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.