· 1 min read
Google's ranking systems reward fast sites, but that's only part of the story. The bigger half is that slow sites quietly bleed conversion — especially on the mid-range Android devices most UK ecommerce traffic actually uses.
The three numbers we budget for
- LCP under 1.5s. The hero must render before doubt sets in. We ship server components, preloaded fonts and AVIF images by default.
- INP under 200ms. Interactivity lag feels like a broken site. Heavy JavaScript — analytics, chat widgets, 3D scenes — loads after interaction, never before.
- CLS under 0.1. Layout shift destroys trust at the exact moment someone reaches for the buy button.
How budgets change engineering decisions
A performance budget is a design constraint, the same as brand colours. It decides which animation library ships, whether that carousel exists, and how the 3D hero degrades on low-power devices. Retrofitting speed after launch costs three times what designing for it costs.
The takeaway
Ask any software house pitching you for their last three Lighthouse scores — on mobile, on the live site, today. If they can't answer, performance was never in their definition of done.