The Real Cost of Poor Web Performance: A Data-Driven Analysis

The CFO did not ask for Lighthouse scores. They asked which line on the P&L moved when the homepage got slower.

That question shows up in agency stand-ups more often than teams admit. Marketing has Search Console exports. Engineering has PSI tabs from Monday. Nobody has one sentence finance can paste into a business case.

Published research keeps returning to the same pattern. Google and Deloitte's Milliseconds make millions work tied small speed gains to measurable funnel shifts in retail. Yottaa's 2025 index reports roughly 3% higher mobile conversions per second saved across large e-commerce samples. Those numbers are retail-heavy, but the shape holds elsewhere: delay shows up in bounce before it shows up in revenue models.

Core Web Vitals give teams a shared vocabulary. LCP for loading, INP for interaction latency, CLS for visual stability. They do not replace your analytics, but they connect engineering work to behaviours that correlate with abandonment.

What we track when building the case internally:

• Map cost to URLs that earn the next step (pricing, checkout, key landers), not only sitewide averages

• Quote CrUX percentiles where coverage exists; use synthetic schedules where it does not

• Segment paid landing pages by speed band before blaming creative

• Translate metrics into journeys ("signup flow INP", "PDP mobile LCP") for client and finance conversations

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