Put the user in one city and the server in another, and watch the round trip. Then switch on the edge cache.
Every request makes the trip about three times (handshake, then the request itself) before the server even starts working. Move the user or the server and watch the bar — no faster server changes the distance.
First-principles estimate (light in fibre at ~200,000 km/s, 1.3× path length, routing overhead, ~40 ms of server work). Real numbers vary with routes and load.
The edge is servers near your users; a CDN is a network of them that keeps copies close, so the trip is short.
MDN: "CDN (Content Delivery Network) is a group of servers spread out over many locations." The point of spreading them out is distance.
Distance is time. A request to a server on the other side of the world has to travel there and back, which adds latency you can feel. A CDN answers from a nearby "edge" server instead of the faraway origin.
Far origin vs nearby edge
How far the request travels
- Faraway origin
- Across the world
- Nearby edge (CDN)
- To a server near the user
How it feels
- Faraway origin
- A noticeable wait
- Nearby edge (CDN)
- Snappy
What it serves
- Faraway origin
- The live app
- Nearby edge (CDN)
- A cached copy kept close
This site runs at the edge (on Cloudflare's network), which is why an article loads quickly whether you're in Sydney or New York. The cached copy is served from a server near you, not from one origin.
But not everything can be cached. Public, identical-for-everyone things (images, CSS, an article) cache beautifully. Anything personal, one user's dashboard, another's orders, must not be served from a shared cache, or you'd hand one person's data to the next. That's the boundary Row Level Security protects.
Try it with the rules off. In the sandbox, put the user and server far apart and watch the round trip climb. Then switch on the edge cache and watch the same request get short. Distance was the whole cost.
Check yourself
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