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Performance

What is a CDN, and does your website need one?

A CDN serves your site from servers close to each visitor. What it speeds up, what it does not, how caching works, and when a small site benefits from one.

A CDN, content delivery network, is a network of servers around the world that store copies of your site's files and serve each visitor from a nearby location. It makes delivery faster, absorbs traffic spikes and attacks, and often adds HTTPS and compression. It does not make a heavy page light, and it only caches what your headers allow.

Published · Last updated

By Michal Měřínský. Audit methodology.

What a CDN is

Without a CDN, every visitor's request travels to your one origin server, wherever it is. With a CDN, requests go to the nearest edge server, which answers from its cache when it can and only asks your origin when it must. A visitor in Sydney then waits for a server in Sydney, not one in Frankfurt.

What it speeds up

  • Static files: images, scripts, styles and fonts, which are the same for everyone.
  • Pages that are the same for every visitor, if you allow them to be cached.
  • The connection itself: TLS handshakes finish closer to the visitor.
  • Delivery under load: traffic spikes hit the edge, not your server.

What it does not fix

  • A page with a huge JavaScript bundle is still slow to run once delivered.
  • Personalised or logged-in pages usually cannot be cached and still go to your origin.
  • A slow backend is still slow for anything not cached.

How caching is controlled

The CDN follows your Cache-Control headers, plus its own rules. Versioned static assets can be cached for a year:

Cache-Control: public, max-age=31536000, immutable

HTML is usually cached briefly, or revalidated, so updates appear quickly. Never cache responses that contain personal data or set session cookies, or one visitor's page can be served to another.

The most dangerous CDN mistake is caching a logged-in page. Check that responses with Set-Cookie or account data are marked private or no-store.

Do you need one?

If you host on Vercel, Netlify, Cloudflare Pages or similar platforms, you already have one. If you run your own server and your visitors are spread across countries, a CDN is usually worth it, and Cloudflare's free plan is a common starting point. A local business whose visitors are all near the server benefits less, though it still gains protection and caching of static files.

Other things CDNs add

  • HTTPS certificates and automatic HTTP to HTTPS redirects.
  • Compression with Brotli or gzip.
  • DDoS protection and a web application firewall.
  • Image resizing and format conversion on some providers. These are also where new problems come from: firewall rules that block real visitors or crawlers, and SSL settings that cause redirect loops.

What a scan can and cannot tell you

WebAuditOS checks whether responses carry Cache-Control headers, whether HTML responses carry a recognized compression header, how fast the server responds and whether HSTS is set, as delivery clues. Cache-Control presence does not establish that a policy is efficient or safe.

It cannot tell which CDN you use, what it caches internally, or how fast pages load for visitors on other continents from a single scan location.

A fast response from the scan's location does not prove a fast response worldwide. Field data from real users shows that.

Short answers

Is a CDN the same as hosting? No. Hosting runs your site. A CDN delivers copies of it.

Is Cloudflare a CDN? Yes, among other things, including DNS and security services.

Can a CDN hurt SEO? Only through misconfiguration, such as blocking crawlers or caching the wrong pages.

Do I need a CDN for a small site? Not always, but most modern hosts include one anyway.

In the product

The checks this maps to

Every check named here runs on every scan, on every plan - what a plan changes is how many pages are crawled, how long the history is kept and what you can do with the result. These are the ids the findings carry, so you can match a line in a report to the explanation above.

Performance

How do you speed up a website?

The changes that make the biggest difference to load time, in the order to try them: server response, images, JavaScript, fonts, caching and third parties.

Hosting and deploy

What is Cloudflare, and what does it do for a website?

Cloudflare sits between visitors and your server as DNS, CDN, firewall and more. What each part does, the free plan, and the settings that often break sites.