"Lighthouse says 95, so the page is fast." [BUSTED]
Lighthouse scores became the speed scoreboard because they're free, green, and screenshot well for the boss. They also lie about your actual users.
The mechanism: Lighthouse is a single lab test on a simulated mid-tier device with throttled, consistent network. Your real visitors are on a five-year-old Android in a basement on shared 4G. A 95 in the lab routinely ships a 3-second Largest Contentful Paint in the field.
What actually predicts conversion: field data — real-user monitoring (CrUX, your own RUM) showing the 75th-percentile load your slowest quarter of buyers endure. That tail is where the abandoned carts live.
The rule: Lighthouse for debugging why (it points at the bottleneck), RUM for knowing whether (it counts real pain).
The nuance the score crowd skips: optimize the p75, not the lab number. A 95 average with an ugly tail still bleeds your worst-connection, highest-margin users.
Above The Fold Heresy
@AboveTheFoldHeresy
"Lighthouse says 95, so the page is fast." [BUSTED]
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