PERFORMANCE · SEO GLOSSARY
Gzip / Brotli Compression
Gzip and Brotli are server-side text compression algorithms that reduce the size of HTML, CSS, and JavaScript files transferred over HTTP — typically 60–80% smaller than uncompressed. Enabling compression is one of the simplest server-level performance improvements and reduces Time to First Byte (TTFB) for text resources.
Definition
When a browser makes an HTTP request, it sends an Accept-Encoding header listing compression algorithms it supports (e.g., Accept-Encoding: gzip, deflate, br). The server can compress the response body using a matching algorithm and indicate the compression in the Content-Encoding response header. The browser decompresses the body before parsing — this decompression is fast and CPU-cheap compared to the bandwidth saved. **Gzip**: the long-established standard, supported universally. Achieves 60–80% compression on typical HTML/CSS/JS files. Available in virtually all web servers (nginx: gzip on; gzip_types text/html text/css application/javascript; Apache: mod_deflate with AddOutputFilterByType). **Brotli**: Google\'s newer algorithm, achieves 15–25% better compression than Gzip at equivalent CPU cost. Supported in all modern browsers via the br encoding. Requires explicit server configuration (nginx: brotli_static on; Cloudflare, Fastly, and most CDNs enable Brotli automatically). **What to compress**: text resources — HTML, CSS, JavaScript, JSON, XML, SVG. Do NOT gzip already-compressed binary formats — JPEG, PNG, WebP, AVIF, WOFF2, MP4, ZIP. These are already compressed and re-compressing them wastes CPU with no size benefit. **Compression verification**: check response headers — look for Content-Encoding: gzip or Content-Encoding: br in the response to a request from a browser that sends Accept-Encoding: gzip, br. **CDN compression**: most CDNs (Cloudflare, AWS CloudFront, Fastly, Vercel, Netlify) apply Brotli/Gzip compression automatically at the edge — meaning hosting on a CDN typically makes this a non-issue without any server configuration.
Why it matters for SEO
Uncompressed text resources are a preventable source of slow page loads. A 500KB JavaScript bundle served uncompressed vs. 120KB gzip-compressed means 380KB more data must transfer before the JavaScript can execute — adding hundreds of milliseconds of delay on mobile connections. Enabling compression is one of the simplest and highest-ROI server configurations, particularly for sites self-hosting without a CDN. It directly reduces Time to First Byte (TTFB) for HTML and the download time for CSS and JavaScript, improving FCP and LCP.
How DeepSEOAnalysis checks this
The audit checks Content-Encoding response headers on crawled pages and linked resources to detect whether compression is enabled. Pages serving uncompressed HTML are flagged as a performance optimisation. Real-world performance impact is measured via CrUX field data for TTFB, LCP, and FCP.
Useful tools and resources
GLOSSARY
Related terms
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
Core Web Vitals
Three Google metrics — LCP, INP, and CLS — that measure real-user loading, interactivity, and visual stability.
Read definition →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
Render-Blocking Resources
Render-blocking resources are CSS stylesheets and synchronous JavaScript files in the <head> that prevent the browser from rendering page content until they have fully downloaded and parsed — directly delaying First Contentful Paint (FCP) and Largest Contentful Paint (LCP).
Read definition →performance
Time to First Byte (TTFB)
The time between a browser sending an HTTP request and receiving the first byte of the server\'s response — a measure of server response speed that influences crawl rate and LCP.
Read definition →See how your site scores on Gzip / Brotli Compression.
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