CDN.

CDN stands for Content Delivery Network. It is a network of servers located in different regions that helps websites deliver content more quickly and reliably. Without a CDN, every visitor may need to request images, scripts, stylesheets and other files directly from the website’s main server, known as the origin server. The farther the visitor is from that server, the longer the request may take. A CDN can store copies of suitable website files in edge servers across its network. When someone visits the website, the CDN routes the request through an appropriate edge location. If the requested file is already stored there, the CDN can return it without contacting the origin server. This is called a cache hit. If the file is not available in that location, a cache miss occurs and the CDN may need to retrieve it from the origin before returning it to the visitor and possibly storing a copy for later requests. CDNs commonly cache static files such as images, fonts, CSS and JavaScript. They may also cache HTML pages, API responses or other dynamic content when the website and CDN are configured to allow it. A CDN does more than caching in many setups. Depending on the provider, it may also offer traffic routing, image optimisation, protection against distributed denial-of-service attacks, TLS certificate management and controls for filtering suspicious requests. Using a CDN does not automatically make every website fast. Poor caching rules, large images, slow application code, uncached pages or a slow origin server can still cause delays.

A CDN can reduce the distance that website data travels before reaching a user. This may lower network latency, reduce pressure on the origin server and improve loading performance for visitors in different locations. For example, a website whose origin server is in Europe may use a CDN so that commonly requested files can be served from edge locations closer to users in Nigeria instead of fetching every file from Europe. A CDN can improve Time to First Byte in some situations and support better loading performance. However, Time to First Byte is not itself a Core Web Vital. It is a diagnostic metric that can affect loading measures such as Largest Contentful Paint when the server response is slow. The benefit depends on what is cached, whether the request produces a cache hit, how the CDN routes traffic and how quickly the origin responds when it is needed.