Core Web Vitals
Image Compression and Core Web Vitals: LCP, CLS, and File Size
Images are often the largest visible assets on a page, so they can directly affect Largest Contentful Paint and indirectly affect layout stability.
LCP: the largest visible image
LCP often points to a hero image, product image, or large article cover. If that file is too heavy, the main content appears late, especially on mobile connections.
Compress the LCP image carefully rather than aggressively. It needs to load fast, but it is also the image users notice first.
CLS: reserve image space
Compression does not fix CLS by itself. Layout shift happens when the browser does not know the image dimensions before the file loads.
Always define width and height or use CSS aspect-ratio for fixed-format image slots. This keeps text and buttons from jumping while images arrive.
- Compress for LCP
- Reserve dimensions for CLS
- Use responsive srcset where possible
- Avoid oversized originals
Format strategy
Use WebP as the default modern format. Add AVIF for important pages when testing confirms the image still looks right. Keep JPG fallbacks for systems or feeds that require them.
For transparent UI graphics, compare PNG and WebP. For photos, compare JPG, WebP, and AVIF at the display size, not at the original camera size.
Optimization checklist
Start with the pages that get traffic and the images above the fold. A small improvement on a popular page matters more than perfect optimization of a forgotten archive image.
After deployment, measure again. Real Core Web Vitals depend on device, network, server, CDN, fonts, JavaScript, and image weight together.
- Find largest images
- Resize to display dimensions
- Convert to WebP or AVIF
- Set width/height or aspect ratio
- Measure after publishing