Performance

Lighthouse is a smoke test

A 100 means the lab conditions passed. It says nothing about your reader's bus ride. Field data is the truth; the lab is the alarm.

Intermediate1 min read (computed · recorded 7)updated 2026-09-12lighthousevitalsmeasurement

by Motif Editors

revised 2026-09-12 — First publication in bank-3 batch 4.

Part 3 of 3 in Performance arithmetic← The cost of one dependency

Key takeaways

— The lab catches big fires cheaply and early.
— Field data answers the bus ride, the warm cache and the second visit.
— A missing measurement labelled as missing beats an estimated one.

What the lab measures

A throttled, cold, single-visit render of one URL. It catches the big fires: render-blocking everything, a 4 MB hero, fonts that block. That is a smoke test — cheap, early, life-saving.

What it cannot know

The bus ride: a warm cache dying at a tunnel. The second visit: a service worker you shipped last month. The real hero: whatever image the reader's timezone loaded. Field data answers those; the lab never will.

This site ships without field numbers and says so on the perf pages — the plan for collecting them is written, the simulation of them is not. A missing measurement labelled as missing is honest; an estimated Lighthouse is neither.

Practise the lesson

Theory sticks when you ship it. These original Motif assets put this guide's lesson to work — open one and copy it into your own page.

Shake-on-error Fieldpractice Particle Drift Fieldpractice Expanding Search Fieldpractice

Keep learning

All guides Try the interactive Lab