A large image is immediately visible but starts loading late. Before reducing its visual quality further, inspect when the browser discovers the resource. loading="lazy" is often unsuitable for an important image in the initial viewport.
LCP (Largest Contentful Paint) measures when the largest visible image or text content appears. The measured element can differ between devices. Desktop and mobile measurements may identify different LCP elements. Inspect the actual page rather than inferring the result from design-file dimensions.
Check resource discovery
Identify the LCP element in browser performance tools, then locate its image request. Is the file discoverable in initial HTML, only through CSS or later through JavaScript?
A simple immediately visible image might use:
<img src="/images/hero.webp" width="1200" height="750"
alt="Product view" fetchpriority="high">
These example dimensions must match the image’s actual aspect ratio. High fetch priority does not replace suitable file sizes or an early-discoverable address. Do not assign high priority to every thumbnail.
Compare the same mobile view before and after the change with browser caching disabled. Record discovery timing, file size and the LCP element. One faster laboratory run does not establish an improvement for real visitors. Recheck whether a theme or CMS update automatically marks important images as lazy again. The loading-time analysis helps distinguish delivery issues from image design.