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Page caching in WordPress

A page cache keeps finished copies of your pages. The first time someone visits a page, WordPress builds it as usual and the cache stores the result. The next visitors get that stored copy directly, without WordPress having to run again.

Why it matters

Section titled “Why it matters”

Building a page is the biggest piece of work the server does for a visit: PHP starts, loads the plugins and the theme, asks the database for the content and assembles everything. A cached copy skips all of that. The visitor gets the page sooner, which helps Time to First Byte and every metric after it, and the server has far less to do, so it handles more visitors at once.

Not every visit can get a cached copy. Logged-in users, the cart and checkout of a shop, search results and pages with personal content need a page built for them. These visits stay as slow as WordPress is without a cache.

What influences it

Section titled “What influences it”

Cache: The most common reason a visitor doesn’t get a cached copy is that none exists at that moment. Copies expire after a set time, and many caches are cleared when content changes or a plugin is updated. The first visitor after that waits for WordPress again. Cookies and parameters in the address decide whether a visit can share the stored copy at all.

WordPress: WordPress doesn’t include a page cache of its own. A plugin provides one, or the hosting provider does on the server. Plugins that personalize pages or set cookies for every visitor can keep the cache from working.

Server: A page cache on the server answers before PHP even starts, which is faster than a cache that runs inside WordPress.

CDN: A CDN can keep copies of whole pages close to the visitor, so many requests never reach your server.

In Permo

Section titled “In Permo”

The Page cache card is in System info, tab WordPress, under Caching. Permo looks for the common cache plugins among the active plugins, by their plugin file:

  • Green with the plugin’s name when one is active.
  • Yellow when two or more are active, with Make sure only one caches pages.
  • None without a color when there’s none, with No page cache plugin found.

A page cache that the hosting provider, the web server or a CDN runs isn’t a plugin, so the card doesn’t see it. None therefore doesn’t mean that no cache answers your visitors.

Permo’s own measurement of the uncached server response time goes around the page cache on purpose. It shows how long WordPress takes for the visits a cache can’t help.

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Sources