LCP image byte weight and the Core Web Vitals

Sites with an LCP image between 40 and 100 KB LCP image byte weight have a good LCP most often: 88%, against 86% of all sites. Sites with an LCP image over 300 KB hero image size have it least often, at 84%.

An LCP image between 100 and 300 KB is the most used: 26.2% of sites. An LCP image between 40 and 100 KB is 23.9% of sites.

Across the four LCP sub-parts in Chrome's field data, the largest gap between an LCP image between 40 and 100 KB and an LCP image over 300 KB sits in time to first byte (TTFB): 543 ms median for an LCP image between 40 and 100 KB against 663 ms for an LCP image over 300 KB. Overall median field LCP is 1.4s for an LCP image between 40 and 100 KB (TTFB 543 ms, load delay 285 ms, load duration 111 ms, render delay 203 ms) versus 1.5s for an LCP image over 300 KB.

On WordPress, 30% of sites use an LCP image over 300 KB LCP image byte weight, and 76% of WordPress sites have a good LCP, against 86% of all sites. On Shopify, 41% of sites use an LCP image between 40 and 100 KB, and 97% of Shopify sites have a good LCP.

PlatformLCP image byte weight on its sitesGood LCP

Sites with LCP image byte weight 128,914 sites100 to 300 KB hero image 26%Under 40 KB hero image 26%Over 300 KB hero image 24%40 to 100 KB hero image 24%86%

Shopify 7,747 sites40 to 100 KB hero image 41%Under 40 KB hero image 40%100 to 300 KB hero image 15%97%

Squarespace 1,336 sites40 to 100 KB hero image 32%100 to 300 KB hero image 30%Under 40 KB hero image 25%Over 300 KB hero image 13%94%

Drupal 2,319 sitesUnder 40 KB hero image 27%100 to 300 KB hero image 26%40 to 100 KB hero image 24%Over 300 KB hero image 24%93%

Wix 4,494 sitesUnder 40 KB hero image 60%40 to 100 KB hero image 33%92%

Joomla 1,881 sites100 to 300 KB hero image 33%Over 300 KB hero image 30%40 to 100 KB hero image 19%Under 40 KB hero image 18%92%

Adobe Commerce 829 sites100 to 300 KB hero image 31%Under 40 KB hero image 27%40 to 100 KB hero image 23%Over 300 KB hero image 19%84%

WordPress 50,607 sitesOver 300 KB hero image 30%100 to 300 KB hero image 29%40 to 100 KB hero image 22%Under 40 KB hero image 19%76%

100 to 300 KB hero image Under 40 KB hero image Over 300 KB hero image 40 to 100 KB hero image

Does the platform explain the LCP image byte weight difference?

On WordPress, 77% of sites with an LCP image between 40 and 100 KB LCP image byte weight have a good LCP, against 76% of WordPress sites with an LCP image between 100 and 300 KB. Within WordPress, sites with an LCP image between 40 and 100 KB and an LCP image between 100 and 300 KB have a good LCP about equally often. On Shopify, 96% of sites with an LCP image between 40 and 100 KB LCP image byte weight have a good LCP, against 97% of Shopify sites with an LCP image between 100 and 300 KB. Within Shopify, sites with an LCP image between 40 and 100 KB and an LCP image between 100 and 300 KB have a good LCP about equally often.

How intrinsic versus rendered megapixels waste hero bandwidth

The median LCP image is 0.23 intrinsic megapixels rendered into a 0.12 megapixel slot. Serving unresized desktop banners to mobile viewports inflates hero transfer bytes and extends LCP load duration. Across 32,981 sites (25.6%) with a hero image under 40 KB, 87% pass field LCP (1.4s median), dropping to 84% (1.5s median) on the 24.3% (31,381) exceeding 300 KB.