Why your website is slow: the five causes we see most often

This is the single most common cause, by a wide margin. A photograph exported straight from a camera or a stock site can be three or four megabytes. On a mid-range phone on mobile data, that alone can push the moment your page becomes readable past four seconds.

The fix is not complicated: resize the image to the dimensions it is actually displayed at, compress it, and serve it as WebP or AVIF. A hero that was 3.8 MB routinely comes back under 200 KB with no visible difference.

Test it on your phone, on mobile data, with the page not already cached. Testing on a fast laptop on office broadband is how this problem survives for years.

A script in the head without defer or async stops the browser drawing anything at all until it has downloaded and run. Several of them stack up, and the visitor stares at a blank white screen the entire time.

Most scripts do not need to block. Analytics, chat widgets and tag managers can almost always be deferred. The rule of thumb: if the page would still make sense for a second without it, it should not block rendering.

Sites accumulate these. A heatmap tool trialled two years ago, an ad pixel from a campaign that ended, a chat widget from a vendor you no longer use. Each one costs download time, parse time and battery on every single visit.

Open your page source and list every third-party domain being loaded. For each one, ask who looks at the data it produces. If the answer is nobody, delete it. This is the rare optimisation that costs nothing and requires no expertise.

This is measured as Cumulative Layout Shift, and it is why people tap the wrong button or lose their place mid-sentence. It usually happens because images have no width and height, so the browser does not reserve space until the file arrives.

Setting explicit dimensions on every image and reserving space for anything that loads late — ads, embeds, banners — fixes most of it. It is tedious rather than difficult.

A marketing page that ships a megabyte of framework code to render text and a button is paying a heavy cost for nothing. This is harder to fix than the others, because it is usually a consequence of how the site was built.

If you are on a page builder or a heavy theme, the realistic wins are elsewhere first. Fix the images and the blocking scripts before considering a rebuild.

Google PageSpeed Insights names the specific cause and estimates the saving for each. That estimate matters: it tells you whether a fix is worth an afternoon or worth ignoring.

Our performance analyzer reports the same measurements, sized by the saving Google itself measured, alongside the rest of your audit — so a slow page is ranked against your other problems rather than viewed in isolation. Sometimes speed is not actually your biggest issue, and it is useful to know that before spending a week on it.