Most agencies treat performance as a checkbox: run Lighthouse once before launch, screenshot the score for the deck, move on. That's backwards. Performance isn't a report card — it's a conversion lever, and it compounds every single day the site is live.
The mechanism, not the metric
A 100ms delay in load time can measurably drop conversion. This isn't folklore — it's been measured repeatedly by companies with enough traffic to isolate the variable. The mechanism is simple: every extra second between a click and a usable page is a second where the visitor's intent decays. Mobile makes this worse — flaky connections turn a "fast enough on my office wifi" site into a multi-second wait on a real device in the field.
Google's Core Web Vitals (LCP, INP, CLS) aren't arbitrary either. They're proxies for three real user complaints:
- LCP (Largest Contentful Paint) — "why is this taking so long to show me anything?"
- INP (Interaction to Next Paint) — "why does tapping this button feel broken?"
- CLS (Cumulative Layout Shift) — "why did the button move right as I tapped it?"
Fix the metric, and you've fixed the actual complaint. That's why we treat 90+ mobile PageSpeed as a floor, not a stretch goal.
Where the score actually comes from
A high score isn't a plugin you install after the fact — it's a sequence of decisions made throughout the build:
- Image strategy first. Every raster image ships as AVIF/WebP through
next/image, sized for its actual container, with explicit dimensions so nothing shifts on load. - Ship less JavaScript. Heavy interactive pieces (canvases, complex animation) are code-split and lazy-loaded, only paying their cost when they're actually needed — and paused when they're off-screen.
- Respect the font. Self-hosted or
next/font-loaded type, preloaded, withfont-display: swapso text never sits invisible waiting on a webfont. - Measure on a mid-tier device, not your laptop. A site that feels instant on a 2024 MacBook can be unusable on the actual mid-range Android phone most of the world browses on.
None of this is exotic. It's just discipline applied consistently, from the first commit — which is exactly the point: performance is cheap to build in and expensive to retrofit.