CDN

A CDN is a network of servers distributed around the world that hold copies of your static files — images, stylesheets, scripts, video — and serve each visitor from a location near them. A request from Sydney is answered in Sydney rather than crossing the planet to your server and back.

The gain is latency, and it is large: distance is a fixed cost that no amount of server optimisation reduces. A CDN also absorbs traffic that would otherwise reach your infrastructure, which is why sites survive being linked from somewhere popular.

The thing to get right is cache invalidation. Files are cached at the edge for as long as you tell them to be, so a deployment can leave visitors on the old stylesheet for a day unless filenames change when their content does.