Cache.

Caching happens on several levels of a website at the same time. The browser cache stores images, fonts, stylesheets and scripts locally on the visitor's device; on the second page view, these files do not have to be downloaded again. A CDN keeps copies of the website on servers close to users. On the web server itself there are page, object and database caches that prevent the same page from being reassembled on every request.

Browser and CDN caching is controlled via HTTP headers, above all Cache-Control. A value such as 'max-age=31536000, immutable' tells the browser that a file will remain unchanged for a year, which makes sense for images and scripts with a hash in the file name that receive a new name with every change. HTML pages, by contrast, are usually cached briefly or not at all so that new content is visible immediately. Modern frameworks combine the two: statically generated pages sit ready in the cache and are rebuilt in the background when content changes.

For loading time, caching is one of the biggest levers. A server that delivers a finished page from the cache responds in milliseconds rather than seconds; this improves time to first byte and thus, indirectly, the Core Web Vitals. On WordPress sites, a caching plugin is often the difference between a sluggish and a usable website, though with the risk of plugins getting in each other's way.

The downside is the familiar sentence 'I can't see the change'. If you edit a page and see the old version in your own browser, you are usually looking at a cached copy. A hard reload, a private window or specifically clearing the server or CDN cache solves this. Well-configured caching makes exactly this situation rare, because HTML is cached briefly and static files for a long time.