A website can look fine to the person who owns it and still frustrate the people trying to use it. Pages that take a moment too long to settle, buttons that shift just as you reach for them, taps that don't register straight away. None of that shows up when you glance at your own homepage, but Google notices it, and so do visitors who leave before they've seen what you actually offer.
Why speed and stability affect more than just impressions
Search engine optimisation (SEO, the practice of shaping a site so it ranks well in search results) used to be mostly about words and links. Google now also looks at how a page behaves once someone lands on it, as one signal among several that feed into ranking. A site that loads slowly or jumps around under a visitor's finger tends to lose people before they convert, whatever its content says. That matters for organic traffic (visitors who arrive through unpaid search results), because a slow page can quietly undercut work you've already put into keyword research and content.
The three measurements Google uses for this are called Core Web Vitals. They're not a judgement on your writing or your offer. They measure the mechanics of the page itself, and they're worth understanding on their own terms before you chase a number.
What Core Web Vitals actually measure
There are three of them, and each one tracks a different kind of friction a visitor can hit.
Largest Contentful Paint
Largest Contentful Paint (LCP) is the time it takes for the biggest visible element on the page, usually a hero image, a heading, or a block of text, to finish rendering. It's the closest thing to a visitor's own sense of "has this page loaded yet". A hero image that's far larger than it needs to be, or a font that blocks rendering while it downloads, are common reasons this drags.
Interaction to Next Paint
Interaction to Next Paint (INP) measures how long the page takes to respond once someone clicks, taps, or types. A page can finish loading and still feel sluggish if every button press has a noticeable lag before anything happens. This usually comes down to scripts running on the page competing for the browser's attention at the wrong moment.
Cumulative Layout Shift
Cumulative Layout Shift (CLS) tracks how much the page's layout moves around as it loads. The classic case is an image or an advert slot that has no reserved space, so the text above it jumps downward the instant the image arrives, right as someone was about to tap a link. It's the metric most likely to cause an accidental click on the wrong thing.
Checking where your pages stand
Google Search Console, a free tool from Google that shows how your site performs in search, reports Core Web Vitals data for pages it has already indexed, using real visits over time. That's useful, but it won't tell you anything about a page nobody has visited yet, and it can take a while to build up enough data to show a pattern.
A Core Web Vitals Checker gives you a faster read on a specific page right now. It's a useful second opinion, not a verdict: a single automated check can return a different result from one run to the next depending on network conditions and server load at that moment, so treat any one reading as an indication rather than a fixed score, and re-check a page that matters before you act on it.
What usually causes a poor reading
Most problems trace back to a small handful of causes.
- Images uploaded at their original size
- Fonts and scripts loaded from third-party services that are slow to respond
- Adverts, embeds, or cookie banners that insert themselves into the page after the rest has already rendered
- Heavy JavaScript running on page load, often from plugins added one at a time without anyone checking the cumulative cost
None of these are unusual. They tend to build up gradually as a site gains plugins, embeds, and tracking scripts.
Fixing it without rebuilding the site
You don't need to start again. Compressing and properly sizing images usually gives the biggest single improvement to loading time, because images are often the heaviest thing on a page. Reserving space for anything that loads in later, such as an image or an embedded video, stops the layout jumping around it. Removing plugins or scripts you no longer actually use reduces the load on every single page at once, which is often more effective than optimising one page in isolation.
Work through pages in order of how much traffic they get. A slow checkout or landing page affects far more visitors than a slow page buried three levels into the site, and it's the one worth your time first.
Where this fits into promoting your site
Page speed is one input into a much larger picture of how a site gets found and used. If you haven't yet worked out which channels are worth your time for a site at your stage, the Promotion Channel Prioriser walks through a short set of questions and returns a shortlist suited to your budget and time. For the fuller sequence, from the groundwork before launch through to ongoing promotion, the Website Promotion Playbook sets it out chapter by chapter.