0.1 seconds is worth 8.4% of your conversions

Performance arguments usually stall because nobody can put a number on them. Deloitte Digital, commissioned by Google, put a number on them.

Where the milliseconds actually go

• Unoptimised images — still the single largest payload on most marketing sites
• Render-blocking third-party scripts: chat widgets, heat maps, four analytics tools measuring the same thing
• Client-side rendering that makes the browser fetch data before it can paint anything
• Web fonts loaded without a swap strategy, delaying first meaningful paint
• No caching or CDN, so every visitor pays the full origin round trip

The fixes, in order of payback

Start with an audit of what actually loads on a real device on a real network, not a desktop on office wifi. Then: serve modern image formats at the right dimensions, defer or remove third-party scripts you cannot name a use for, render on the server so the first paint contains real content, and set a performance budget that fails the build when it is exceeded.

That last one matters most. Speed regressions are not fixed by a one-off optimisation project; they are prevented by making performance an acceptance criterion, the same way you treat a failing test.

What good looks like

Largest Contentful Paint under 2.5 seconds on mobile, Interaction to Next Paint under 200 milliseconds, and Cumulative Layout Shift under 0.1 — measured on field data from real users, not a lab score. Hit those and you have collected most of the revenue Deloitte measured.

Sources

• web.dev — Milliseconds make millions (case study)
• Deloitte Ireland — Milliseconds Make Millions