Latency
Definition
Distance sets the floor. Light travels about 200,000 km per second in fibre, and real routes are not straight, so a rough working figure is 1 ms of round trip for every 100 km. London to Frankfurt is typically 12 to 20 ms, London to New York around 75 ms, London to Sydney around 250 ms. Mobile adds its own cost before the packet reaches fibre at all: 30 to 60 ms on 4G, sometimes far more on a congested cell. What makes latency expensive on the web is that protocols need several round trips before any content moves — a DNS lookup, a TCP handshake, then a TLS handshake. Each of those multiplies the same number, so a 250 ms link can cost well over a second before the first byte of HTML appears.
Why It Matters
Buying more bandwidth does nothing for latency, which is why a fast office connection can still feel sluggish against a distant server. Consider a visitor in Sydney loading a page from a server in London: roughly 250 ms per round trip, times four for DNS, TCP, TLS and the request itself, is a second of waiting before anything renders. Moving the same file to an edge node in Sydney cuts each round trip to perhaps 10 ms and the whole sequence to under 100 ms, with no change to the file. This is the single largest reason static content is worth putting on a CDN — the win comes from geography, not from clever engineering.
How It Works
Reducing latency means either shortening the distance or needing fewer trips across it. Shortening the distance is what a point of presence does: copies of your files sit in dozens of cities, and a visitor is routed to a nearby one. Cutting round trips is the job of the protocols — TLS 1.3 completes its handshake in one round trip instead of two, HTTP/2 carries many requests on one already-open connection, and connection reuse avoids repeating the handshake for every asset. Preconnect hints let the browser start a handshake to a third-party host before it knows it needs one. Caching removes the trip entirely: a response still valid under its Cache-Control header costs zero milliseconds because it never leaves the device.
Real-World Example
A training company publishes a 3 MB slide deck on 99helpers at onboarding-week1.99helpers.site and shares it with staff in Manchester, Toronto and Singapore. The Manchester team sees the first byte in about 15 ms, and because the same file is cached at an edge node near each office, Toronto and Singapore see roughly 20 and 25 ms rather than the 90 and 260 ms a single UK server would have imposed. Nothing about the file changed; only how far it had to travel.
Common Mistakes
- ✕Confusing latency with bandwidth — a gigabit line to a server 15,000 km away still waits 250 ms for every round trip
- ✕Testing only from a machine near the origin, which makes a globally slow site look fine on every check you run
- ✕Adding third-party scripts from several new hosts, each of which needs its own DNS, TCP and TLS sequence before it returns anything
- ✕Chasing file size when the real cost is trip count — ten small sequential requests can beat one large parallel download on a high-latency link
Related Terms
CDN
A content delivery network: a set of servers spread across the world that keep copies of your files and answer each visitor from somewhere close to them. It is the service; the machines it runs on are its edge network.
Edge Network
The fleet of servers a delivery provider runs in cities around the world, positioned close to visitors rather than close to your origin. It is the machinery a CDN is built from.
Point of Presence
A single physical location in a delivery network — the racks of servers in one city that answer requests from that region. Shortened to PoP, it is the smallest unit an edge network is counted in.
Time to First Byte
Time to First Byte, or TTFB, is the gap between a browser starting a request and the first byte of the response body arriving. It measures everything that has to happen before the page can begin — not how long the page takes to finish.
Serverless
A hosting model where you deploy a function rather than a machine, and the provider starts it on demand and charges for the time it ran. There are still servers — you simply do not choose, patch or keep them.
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