How CDN works

Network latency (the time it takes for a request to travel from the user to the server and back) depends on physical distance. A server in Europe responds in ~30ms to a European user, but in ~200ms to a user in Asia or the Americas. A CDN reduces that latency by serving content from edge servers distributed worldwide.

Modern CDNs do not only cache static files (images, CSS, JavaScript). They can also cache HTML responses, execute logic at the edge (edge computing), automatically optimise images, apply compression and manage SSL certificates. They are a critical piece of any web architecture that prioritises performance.

When a user visits your website, the request is routed to the nearest edge server (PoP — Point of Presence). If that server has a cached copy of the requested resource (cache HIT), it returns it immediately without contacting your origin server. If it does not (cache MISS), it requests the resource from the origin, returns it to the user and stores a copy for future requests.

Major CDNs (Cloudflare, AWS CloudFront, Fastly, Akamai) have hundreds of PoPs worldwide. Cloudflare operates in over 310 cities, meaning virtually any user is less than 50ms from an edge server.

Cache configuration is where a CDN adds or subtracts value. A cache that is too aggressive serves stale content; one that is too short generates many MISSes and does not leverage the distribution network. The key is to define appropriate TTLs per resource type.

Versioned static assets (main.a1b2c3.js, styles.x7y8z9.css) can be cached indefinitely (1-year TTL) because the filename changes when the content changes. HTML should have a shorter TTL (seconds to minutes) to reflect content updates. Images are typically cached for days or weeks.

A well-configured CDN can reduce Time to First Byte (TTFB) from 200–500ms to under 50ms. This directly impacts Largest Contentful Paint (LCP) and the user’s perception of speed. Google uses Core Web Vitals as a ranking factor, making the CDN a technical SEO tool.

Beyond caching, modern CDNs apply automatic optimisations: Brotli/gzip compression, HTTP/2 and HTTP/3, Early Hints, image optimisation (WebP, AVIF, automatic resizing) and HTML/CSS/JS minification. Each optimisation reduces transferred bytes and improves load times.

Cloudflare is the most popular thanks to its generous free tier, its 310+ PoP network and additional features (WAF, DDoS protection, Workers for edge computing). AWS CloudFront integrates natively with the AWS ecosystem. Fastly stands out for its programmable cache with VCL/Compute@Edge and purge times under 150ms.

Vercel and Netlify include an integrated CDN in their deploy platform, ideal for JAMstack/headless projects. For high-traffic ecommerce, Akamai and Fastly offer the performance and SLA guarantees required by stores with millions of visits.

Minimum CDN configuration includes: defining the origin (your server or hosting service), configuring HTTPS with an SSL certificate (most CDNs manage this automatically), setting appropriate cache headers and verifying that compression is active.

Advanced best practices include: using custom cache keys for dynamic content, configuring stale-while-revalidate to balance freshness and performance, implementing automatic purge when new content is deployed, and monitoring cache hit ratio (target: >90%).