Medium impactPerformance
Images Not Optimized
Oversized images are the #1 cause of slow LCP. Here's how to compress, convert, and serve them right.
What it means
Images are served at larger dimensions or file sizes than needed — often 5–10x what the page actually displays.
Why it matters
Image weight directly hits LCP, which is a Core Web Vitals signal and a real UX issue on mobile.
How to fix it
- Identify images flagged as oversized.
- Resize to the actual display dimensions (at most 2x for retina).
- Convert to WebP or AVIF.
- Set width and height attributes to prevent layout shift.
- Lazy-load below-fold images (loading="lazy").
- Preload the LCP image with fetchpriority="high".
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Frequently asked questions
What format should I use?
WebP is the safe default in 2026 — supported everywhere. AVIF is smaller but slightly slower to decode.