Content Delivery Network

A CDN is a network of servers spread across geographic locations that keeps a copy of a site's content close to the user, so distance — and therefore retrieval time — is reduced.

A CDN's logic is simple: even data takes time to travel, so the closer the server answering the user is, the faster the connection and retrieval stages get. Its effect is seen most on static files (images, scripts, fonts) and on the early stages of the chain (connection and time-to-first-byte), not necessarily on code built dynamically each time. A common trap is treating a CDN as a magic fix for all slowness; if the root cause is a database query or heavy JavaScript, a CDN does not help. In Iran there is a practical point too: a domestic CDN with points of presence near in-country users usually outperforms a foreign CDN whose route comes from outside, and that difference often shows up in a per-operator breakdown of network time.