LCP image byte weight and the Core Web Vitals
25.7% of audited sites (31,128) keep their LCP hero image under 40 KB and pass LCP at 84% (1.5s median), whereas the 24.3% shipping hero images over 300 KB drop to 77% (1.7s median).
23.8% of sites use an LCP image between 40 and 100 KB LCP image byte weight, and 85% of those sites have a good LCP, against 82% of all sites.
LCP pass rate
Sites with an LCP image between 40 and 100 KB LCP image byte weight have a good LCP most often: 85%, against 82% of all sites. Sites with an LCP image over 300 KB hero image size have it least often, at 77%.
LCP image byte weight usage
An LCP image between 100 and 300 KB is the most used: 26.1% of sites. An LCP image between 40 and 100 KB is 23.8% of sites.
The four LCP phases
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): 631 ms median for an LCP image between 40 and 100 KB against 799 ms for an LCP image over 300 KB. Overall median field LCP is 1.5s for an LCP image between 40 and 100 KB (TTFB 631 ms, load delay 316 ms, load duration 139 ms, render delay 230 ms) versus 1.7s for an LCP image over 300 KB.
Understanding LCP image byte weight
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 31,128 sites (25.7%) with a hero image under 40 KB, 84% pass field LCP (1.5s median), dropping to 77% (1.7s median) on the 24.3% (29,464) exceeding 300 KB.
Compress hero images below 100 KB using AVIF or WebP and serve responsive srcset widths matched to the rendered viewport slot. Across all audited sites, 25.7% keep the hero image under 40 KB (84% good LCP, 1.5s median) versus 24.3% above 300 KB (77% good LCP, 1.7s median).
How LCP image byte weight plays out per platform
What the field data shows when you isolate a single CMS or framework.
WordPress 47,685 sites 30% of WordPress sites ship an LCP hero image heavier than 300 KB (66% vs 72% good LCP at 40 to 100 KB).
Across WordPress sites, 30% serve a hero image over 300 KB (66% LCP pass rate, 2.1s median) compared with 72% (1.9s median) on the 21% with a 40 to 100 KB hero image.
React 6,487 sites React hero images over 300 KB pass LCP 28 points less often than hero images under 40 KB.
Across React sites, 48% serve an LCP image under 40 KB (90% LCP pass rate, 1.3s median), 29% serve 40 to 100 KB (89%), and 9% exceed 300 KB (62% pass rate, 2.2s median).
By Arjen Karel · 212,308 crawled sites (197,524 with all devices Chrome field data, 121,027 sites with LCP image byte weight, October 2026). How we measured.