Two costs, not one
A slow page costs you twice. Conversion rate falls, and quality scores in paid auctions fall with it, so you pay more per click for the traffic that converts less.
On one commerce replatform we measured a 0.4% to 2.9% conversion rate move alongside a 22% drop in effective cost per click. Neither number was the goal; both came from the same work.
Largest Contentful Paint is mostly images
In practice most LCP problems are one hero image served at the wrong size in the wrong format from the wrong origin. Fix the image pipeline before touching JavaScript.
Interaction to Next Paint is mostly third parties
Tag managers, chat widgets, session recorders and consent banners compete for the main thread at exactly the moment a user tries to interact. Audit what is loading, defer everything non-critical, and delete the three tools nobody has opened this quarter.
Measure field data, not lab scores
A perfect Lighthouse score on a developer laptop means nothing. Real user monitoring on real devices is the only number worth reporting.