When a Shopify product page feels slow on mobile, start by looking at what the browser loads and when. Here are three checks you can run before changing the theme.

1. Find the LCP element

Run a mobile performance test and identify the Largest Contentful Paint (LCP) element. On a product page it might be the featured image, but the report will tell you which element actually counted for that run.

If it’s the main product image, make sure it appears in the initial HTML and loads eagerly. Consider fetchpriority="high" for that image when it’s the main LCP candidate. Keep offscreen gallery images lazy-loaded.

2. Check which image mobile visitors download

Open DevTools, select a mobile viewport, and reload with the Network panel open. Find the product image request and compare the downloaded image width with the space it occupies on screen, accounting for device pixel ratio.

In Liquid, use Shopify CDN image widths in your responsive markup.…