Images: the number one cause of slow websites

The size problem

Modern phones produce photographs of several megabytes at four thousand pixels wide. Uploaded directly, that file is sent to every visitor, then scaled down by their browser to display at perhaps eight hundred pixels.

The visitor downloaded roughly twenty times more data than they needed, and on mobile data they waited for it. Ten such images on a page is a site that takes many seconds to load, as covered in site speed and Core Web Vitals.

Resizing before upload is the single highest-return action available on most small business sites.

What to actually do

In order of impact.

• Resize to display sizeA full-width image rarely needs to exceed 1600px wide. A thumbnail needs a few hundred. Match roughly what is displayed.
• CompressQuality around 80% is visually indistinguishable from 100% for photographs and dramatically smaller. Squoosh (opens in a new tab) is free, runs in the browser, and shows you both versions side by side.
• Use a modern formatWebP is well supported and typically 25–35% smaller than an equivalent JPEG. AVIF is smaller still.
• Choose the right formatPhotographs: JPEG or WebP. Logos, icons and anything with flat colour or text: SVG or PNG. A logo saved as a JPEG will look muddy and weigh more.
• Lazy load below-the-fold imagesImages further down the page can load as the visitor scrolls rather than competing with what is on screen.
• Set width and heightPrevents the page jumping around as images load — the layout shift measured by CLS.

Alt text

Alt text describes an image for people using screen readers, and is read by search engines that cannot see pictures.

Describe the content and purpose plainly: "Bathroom refitted with a walk-in shower in Headingley" rather than "IMG_4471" or a list of keywords. Purely decorative images should have empty alt text so screen readers skip them rather than announcing a filename.

Google publishes specific guidance on images (opens in a new tab) if you want the detail. It also matters for accessibility, where images without descriptions are one of the most common failures.

Video, and why to be careful

Self-hosting video is the fastest way to make a site slow. A short clip can outweigh your entire page many times over, and your hosting is not built to stream it.

Upload to YouTube or Vimeo and embed. Better still, embed lazily so the player only loads when someone clicks — an embedded video often loads hundreds of kilobytes of scripts before anyone presses play.

Avoid autoplaying background video on mobile. It burns data, drains battery, and is one of the more reliable ways to make someone close the tab.

Sources and further reading

• Google Images best practices (opens in a new tab)Google Search Central
• Squoosh — image compression (opens in a new tab)Google Chrome LabsFree, browser-based, no upload required.
• Optimize images — web.dev (opens in a new tab)Google web.devThe technical detail behind all of the above.

Common questions

How big should my website images be?

Aim under 200KB for most, and under 500KB for a large hero image. Dimensions should roughly match display size — usually no more than 1600px wide for full-width images.

What is WebP and should I use it?

A modern image format that produces smaller files than JPEG at equivalent quality. Support is universal across current browsers. Most site platforms can convert automatically, often via a plugin.

Do stock photos hurt my SEO?

Not directly. They hurt conversion instead — visitors recognise them and they weaken trust. Real photographs of your work perform better with people, which is what matters. See building trust.