PERFORMANCE · SEO GLOSSARY
Image Compression
Image compression reduces the file size of images while maintaining acceptable visual quality — directly affecting page load time, LCP (Largest Contentful Paint), and bandwidth consumption. Uncompressed or over-sized images are one of the most common causes of poor Core Web Vitals on image-heavy pages.
Definition
Image compression works by removing redundant data from image files. Two types: **Lossy compression**: permanently removes image data to achieve larger file size reductions. JPEG uses lossy compression — quality settings from 60–80 are typically visually indistinguishable from 100 at 70–80% smaller file sizes. WebP lossy compression achieves similar or better quality at 25–35% smaller size than equivalent JPEG. **Lossless compression**: removes metadata and redundant encoding without altering pixel data. PNG uses lossless compression; tools like TinyPNG/PNGQuant apply additional lossless optimisation. **Modern image formats for the web**: (1) **WebP** — Google\'s format, now supported by all major browsers. Provides 25–35% smaller files than JPEG at equivalent quality for photographs; significantly smaller than PNG for graphics with transparency. (2) **AVIF** — newer, even smaller than WebP at equivalent quality. Browser support is now broad (Chrome, Firefox, Safari 16+). (3) **JPEG XL** — promising format but limited browser support as of 2026. Use WebP as the primary format with JPEG fallback for maximum compatibility. **Implementation approaches**: next-generation format conversion at build time (via ImageMagick, Sharp, or build tool plugins); CDN-level auto-format negotiation (Cloudflare Image Resizing, Fastly, Cloudinary — serve WebP/AVIF automatically based on Accept header); CMS media pipelines (Shopify, Contentful, Sanity auto-convert uploads to WebP). **Sizing**: serve images at the size displayed, not larger. A 300px wide thumbnail served from a 2000px wide source image wastes bandwidth. Use srcset and sizes attributes to serve appropriately sized images for each viewport. **Lazy loading**: add loading="lazy" on below-fold images so they don\'t delay initial page load. Do NOT use loading="lazy" on above-fold images — it can delay LCP.
Why it matters for SEO
Images are typically the largest resources on web pages. Unoptimised images are the most common cause of poor LCP (Largest Contentful Paint) scores — the primary Core Web Vitals metric Google uses as a Page Experience ranking signal. A hero image served at 4MB instead of 120KB because it wasn\'t compressed can push LCP from 1.5s to 6s on a 4G connection. Image compression is one of the highest-ROI performance optimisations available because the file size reduction is large and the implementation is mechanical.
How DeepSEOAnalysis checks this
The audit assesses image optimisation signals from the served page HTML: whether images have explicit width and height attributes (prevents CLS from image layout shift), whether above-fold images lack loading="lazy" (which would delay LCP), whether fetchpriority="high" is set on the identified LCP image, and whether images are served in modern formats (WebP or AVIF) via Content-Type response headers. Real LCP values are from CrUX field data.
Useful tools and resources
GLOSSARY
Related terms
core web vitals
Largest Contentful Paint (LCP)
The Core Web Vital measuring how long it takes for the largest visible content element in the viewport to render — good LCP is under 2.5 seconds at the p75 threshold using real Chrome user data (CrUX).
Read definition →performance
Core Web Vitals
Three Google metrics — LCP, INP, and CLS — that measure real-user loading, interactivity, and visual stability.
Read definition →performance
Page Speed
The time it takes for a page to load and become usable — measured by multiple metrics including TTFB, FCP, and Core Web Vitals.
Read definition →performance
Lazy Loading
A browser performance technique where images and iframes are deferred from downloading until they are near the viewport — reducing initial page weight and improving load time for above-the-fold content, but harmful for LCP images when applied incorrectly.
Read definition →performance
Next-Gen Image Formats (WebP, AVIF)
Modern image compression formats — WebP and AVIF — that deliver significantly smaller file sizes than JPEG and PNG at equivalent visual quality, directly reducing image payload and improving Core Web Vitals LCP.
Read definition →See how your site scores on Image Compression.
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