From a URL to a pixel

A DNS lookup turns a name into an address. Your machine asks a resolver, the resolver asks the root, then the registry, then the authoritative nameserver, and caches the answer for as long as the TTL says. Cold, that is anywhere from twenty to a hundred and twenty milliseconds. Warm, it is zero, so measuring your own site after you have loaded it forty times tells you almost nothing about what a stranger experiences.

Then a TCP connection opens, which is one round trip, and TLS negotiates on top of it, which is one more on TLS 1.3 and two on 1.2. Notice the unit: round trips, not milliseconds. A round trip to a server in the same city is about 10ms; to one on another continent it is 150ms or more, and no amount of server-side optimisation changes the speed of light. This is one reason a CDN is not a micro-optimisation, it is the only thing that makes those numbers smaller.

Only now does the actual HTTP request leave your machine. The server thinks for a while and the first byte comes back. That interval is Time To First Byte, and it is the number that separates a slow host from a slow page. A static file on a good CDN answers in 30ms; an unoptimised database query behind a cold serverless function can take two seconds before a single byte of your beautiful HTML has moved.

The response streams in, and the browser starts parsing before it has finished arriving. This is the moment everything changes hands. Everything after this point is yours: how much HTML there is, what it references, in what order, and how much work the browser has to do before it can show anything. Everything before it belongs to your host, your DNS provider and physics.

Drive the waterfall below through all four connection presets. Watch the number at the bottom, the share of total time your code can actually influence. On fibre with a warm cache it is most of it. On a cold 3G connection from the other side of the world it collapses to under a quarter, and the entire performance conversation changes from "make the JavaScript smaller" to "get the bytes closer".

One consequence useful to remember: the first visit and the second visit are different products. Cold, you are fighting DNS, TLS and an empty cache. Warm, none of that exists and your own code is the whole cost. Most developers only ever experience the warm path, on a fast laptop, on the same continent as the server, which is how sites that feel instant in development take eleven seconds in the field.