Part III — The Speed
5 Performance
GoalMeasure what users feel, then fix it in the order that actually moves the number.
- 5.1 Performance Overview Three metrics describe what a user feels: is it there, does it respond, does it stay still.
- 5.2 Performance Importance Latency is a business metric — it moves conversion, retention, and reach, not just Lighthouse scores.
- 5.3 Performance Monitoring Lab tests tell you what changed; field data tells you what users actually got.
- 5.4 Performance Tools Read the waterfall before you read the code — the shape of the chart names the bottleneck.
- 5.5 Network Optimization Fewer round trips, closer bytes, fewer bytes — in that order, because that's the order of impact.
- 5.6 Rendering Patterns Where and when HTML gets built — CSR, SSR, SSG, ISR, streaming — decides your LCP before you write a line.
- 5.7 Build Optimization Ship only the code this route needs, compress it, and let the browser cache it forever.
Capstone
Take a page with a 6-second LCP under 2.5s. List every change you made and the milliseconds each one saved.