Speed Index
Speed Index is a Lighthouse performance metric that measures how quickly the visible content of a page is progressively displayed during page load. It's calculated by analyzing video frames of the page loading and scoring based on how much of the page is visually complete at each point in time. A good Speed Index for mobile is under 3.4 seconds.
Why It Matters for Shopify Stores
Speed Index captures the perceived loading experience — a page that loads content progressively from top to bottom scores better than one that shows nothing for 4 seconds then loads everything at once. For Shopify stores, above-the-fold content (hero image, product photo) should load first. Optimizing Speed Index means prioritizing critical rendering path resources and ensuring your LCP element loads before other below-fold content.
How to Check Your Store
Speed Index appears in the Google PageSpeed Insights / Lighthouse report under 'Performance Metrics'. A filmstrip view shows how the page loads progressively.
Use the free shopify speed test toolHow to Fix It
Optimize LCP first (it's the biggest driver of Speed Index). Eliminate render-blocking resources. Preload hero/product images. Defer below-fold images and scripts.
Related Terms
LCP (Largest Contentful Paint)
Core Web VitalsLargest 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.
FCP (First Contentful Paint)
Core Web VitalsFirst Contentful Paint marks the point when the browser first renders any text, image, or non-white content to the screen.
Render-Blocking Resources
PerformanceRender-blocking resources are CSS or JavaScript files that prevent a browser from displaying page content until they've fully downloaded and parsed.
Lazy Loading
PerformanceLazy loading is a technique where images and other resources are only loaded when they're about to enter the user's viewport — rather than all at once when the page first loads.
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