Development · 9 min read
The measurable cost of a slow page
We instrumented eight client sites for a year. Here's what each additional second actually cost them.
Priya NathanEngineering Director · 22 March 2026
Takeaways
- Performance work has a revenue line, not just a Lighthouse score.
- Third-party scripts are usually the largest single cost.
- Budget milliseconds the way you budget money.
01
The numbers
Across eight ecommerce and lead-gen sites, each additional second of LCP correlated with a 7–12% drop in conversion, worst on mobile networks.
“Every additional second has a price. We finally put a number on ours.”
02
Where the seconds go
Images and third-party scripts, almost always. Tag managers are the most expensive free product in marketing.
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