A 1-second delay in page load time reduces conversions by 7%. That number has been cited in marketing decks since 2017. It is old enough to be a cliché and accurate enough to still be true.
The more important number is what that delay costs over a year. For a site doing £50k/month in conversions, 7% is £3,500 per month. That is £42,000 a year. That is more than most agencies charge for a website.
The slow site is almost never the site that gets the redesign brief. It gets the let's refresh the copy brief. The cost is invisible because no one runs the number.
What Core Web Vitals are actually measuring
Google's Core Web Vitals are not arbitrary SEO requirements. They are proxies for user behaviour that Google measured across billions of sessions.
LCP — Largest Contentful Paint — measures how quickly the main content renders. Users abandon pages that take more than 2.5 seconds to show something meaningful. Not because they are impatient. Because 2.5 seconds of nothing reads as broken.
CLS — Cumulative Layout Shift — measures how much the page moves while loading. A button that jumps 200px when an image loads is a button that gets accidentally clicked. A headline that shifts after a font loads is a headline that reads incorrectly. Layout instability is not an aesthetic problem. It is a trust problem.
INP — Interaction to Next Paint — measures responsiveness. A form field that takes 400ms to register a keypress feels broken. A filter that takes 600ms to update feels slow. Users read latency as unreliability.
Where the cost accumulates
The direct cost is conversion. The indirect cost is brand.
A slow site signals that no one is paying attention. It signals that the organisation does not care enough about its customers' time to invest in their experience. That signal is received before the user consciously registers it. By the time someone notices that a site is slow, the damage to their perception is already done.
The SEO cost is a multiplier. Pages that fail Core Web Vitals rank lower. Lower rank means less traffic. Less traffic means fewer conversions. The slow site pays the performance penalty twice — once in conversion rate and once in traffic volume.
What fixes it
Not a CDN, necessarily. Not a framework change, necessarily.
The most common causes of slow sites are the most boring ones: unoptimised images, render-blocking scripts, no caching strategy, third-party embeds loaded synchronously. These are not hard problems. They are neglected problems.
An image that is 4MB because no one ran it through a compressor. A Google Analytics script loaded in the head because that is where the instructions said to put it. A font loading from an external CDN with no preconnect hint. A video embed that pulls in 200kB of JavaScript before the user has scrolled anywhere near it.
Fix the boring things first. Measure after each fix. Most sites achieve passing Core Web Vitals without touching the architecture.
The budget question
The performance conversation is usually framed as a technical debt conversation. That framing makes it optional. Technical debt is for later. Performance that costs £42,000 a year is for now.
Run the number for your site. Total monthly conversions, times average order value, times 7%, times 12. If the result is larger than the cost of fixing the site, the business case is closed.