Performance

JavaScript Defer / Async

The defer and async attributes on <script> tags change how JavaScript loads relative to HTML parsing. defer loads the script in parallel but executes it after the HTML is fully parsed. async loads and executes as soon as it downloads, potentially interrupting parsing. Both prevent render-blocking.

Why It Matters for Shopify Stores

Any <script> tag in <head> without defer or async blocks the browser from rendering the page until that script downloads and executes. On Shopify stores, many app scripts are injected via ScriptTag API and loaded in <head> without these attributes. Adding defer to non-critical scripts (analytics, chat, reviews) can improve FCP by 1–3 seconds on mobile connections.

How to Check Your Store

View the page source of your store and check <script> tags in the <head>. Count those without async or defer attributes — these are your render-blocking scripts.

Use the free shopify app bloat calculator tool

How to Fix It

Add defer attribute to non-critical scripts. Don't add defer to scripts that other scripts depend on without careful dependency analysis. Shopify theme ScriptTag API scripts can be modified to use defer in your theme code.

Related Terms

Render-Blocking Resources

Performance

Render-blocking resources are CSS or JavaScript files that prevent a browser from displaying page content until they've fully downloaded and parsed.

FCP (First Contentful Paint)

Core Web Vitals

First Contentful Paint marks the point when the browser first renders any text, image, or non-white content to the screen.

LCP (Largest Contentful Paint)

Core Web Vitals

Largest Contentful Paint measures how long it takes for the largest visible content element on a page — usually a hero image or headline — to finish loading.

Unused App Scripts

Shopify

Unused app scripts are JavaScript files that load on every page of a Shopify store from apps that have been removed from the store admin but whose code remains in the theme.

Check your store's JavaScript Defer / Async with our free tools

Get a full audit across all 6 performance categories — including performance — in under 60 seconds.

Run a Free Store Audit