DietPress

Every time somebody visits your site, WordPress queries the database, runs your plugins, builds the page from your theme and sends it. For a visitor who is not logged in, the result is the same every time until you change something, so doing all of that again is wasted work.

A page cache does it once and stores the finished HTML on disk. The next visitor gets that file, skipping the database, the theme and most of the loading time. It is normally the single biggest speed gain a WordPress site can get, and it is also what keeps a small hosting plan standing up when a post does well on social media.

You need one unless your hosting already provides it. DietPress detects the usual managed hosts and tells you when that is the case.

The page cache normally serves its stored copy from PHP, as soon as WordPress has loaded the plugins. That skips the database and the theme, but not the start of WordPress itself, and on a site with many plugins that start is most of what a cached page still costs.

The accelerator removes that start too. On Apache, DietPress writes a block of rules to your .htaccess, marked Page Cache by DietPress, and the server hands out the stored copy by itself, without starting PHP or WordPress at all. That is the fastest a page can be served. On nginx, which does not read .htaccess, the Cache tab gives you the same rules to paste into your server configuration.

Phones are served by the server too. With the separate mobile cache on, each address has a copy built for a phone and one built for a desktop, and the rules hand out the right one with the same test WordPress uses, so what PageSpeed measures on mobile is the served copy. Those pages go out with a Vary header, so a CDN or proxy in front keeps the two apart.

Switch it on once the page cache is working, if your site runs on Apache (with mod_rewrite and mod_headers, which nearly every hosting has) or nginx, which is where it has been measured. LiteSpeed reads the same rules as Apache, and there the test is what tells whether they work. It is tested on your home page the moment you switch it on: if the server does not serve that page by itself, with its headers and for the right scheme, the rules are taken out again and the tab says why. Everything the rules leave alone, a logged in visitor, a URL with a query string, a forced reload, is still served or rebuilt by PHP, exactly as without it.

Stored pages still expire on time, even on a site so well cached that WordPress hardly runs its scheduled tasks: each copy is linked under the name of every hour it is still valid in, and the server only looks for the current one. The one exception is the night the clock of the server goes back, when a copy can be served for up to an hour longer. Those extra names are also why a backup that includes wp-content/cache can grow: exclude that folder, as most backup plugins already do, because a backup that does not keep hard links stores each name as a file.

Partly, and it is worth knowing how. The server answers before any PHP runs, so whatever a security plugin does in PHP does not see those visits: blocking an IP address, rate limiting, a bot filter or an under attack challenge. The rules a security plugin writes to your .htaccess, or to the server configuration, still apply: a bot that those rules block still gets its 403 on a page the server serves, while a client that is only blocked in PHP gets the cached page. The same goes for a site closed at server level, by IP address or with a password: on Apache the stored copies are only reachable through the address of their page, which those rules protect. On nginx, see the next question. Pages are never served this way to a logged in visitor or to one with a cart.

If your site relies on the PHP side of a security plugin for anonymous visitors, leave the accelerator off. Keep in mind that the page cache answers very early without it too, before many security plugins run their checks.

nginx does not read .htaccess, so DietPress cannot write the rules for you. With the page cache on, the Cache tab shows them in four parts, each with a comment that says where it goes in the configuration of your site: two inside the server block, one at the start of the location that handles your pages, and one in the location that passes PHP files to PHP-FPM. Paste them, check the configuration with nginx -t, reload nginx and then switch the accelerator on, which tests them on your home page before it stays on. The rules also close the cache folder, which nginx would otherwise let anyone read by its address. One thing to check before pasting: access rules inside your location / block (allow, deny, auth_basic or auth_request) do not reach the pages served from the cache, because nginx sends the request on before it looks at them. Move them up to the server block, or repeat them inside the location of part 2, as its comment says.

If you later change an exclusion, the cookies that keep visitors out, the separate mobile cache or how long browsers keep pages, the tab says the rules changed: paste them again and reload nginx. On a managed hosting where you cannot edit the nginx configuration, leave the accelerator off; the page cache keeps working through PHP.

Open a page in a private window and look at its response headers in the developer tools of your browser: X-DietPress-Cache: HIT-STATIC means the server served it without PHP, HIT means PHP served it from the cache, and MISS means it was built. The first visit to a page after it changes is always a MISS. “Test the cache now” in the Cache tab does the same check for you on the home page. It switches the accelerator off if the server fails that page where the accelerator serves it (with an error, with a copy missing its headers, or with something other than the page DietPress stored) while WordPress still builds it fine. When the accelerator is switched on but cannot run, the test names the reason, and if the server is still answering from the cache by itself, it takes away the names the server finds the stored pages under.

On a local WordPress 7.1 install with Astra, WooCommerce and 29 active plugins, on PHP 8.5 and Apache. Each URL was requested twice to warm up and then fifteen times straight to the web server, and the median was taken; the visits per second come from 300 requests with ten in flight at a time, with the security plugin of the site told not to rate limit them. The only things that changed between the three columns were the page cache and accelerator switches, on the same content, in the same session.

You can repeat it on your own site with any tool that reports time to first byte. From a terminal, curl -o /dev/null -w "%{time_starttransfer}\n" https://yoursite.com/ a dozen times with the cache off, then a dozen with it on, and compare the medians rather than any single reading. Do it while logged out, because your own visits are never cached.

Two honest caveats. A local install has no network latency, so the absolute milliseconds are lower than you will see in production, where the saving is usually larger rather than smaller. And these numbers say nothing about a Lighthouse score: that measures mostly what the browser does with your theme and your scripts after the page arrives, which is a different problem from how fast your server answers.

A browser will not paint anything until it has downloaded every stylesheet in the page. Those files are render-blocking: your visitor stares at a blank screen while they arrive.

Critical CSS is the small subset of rules needed to draw what fits on the screen at first sight. DietPress writes it straight into the page, so the browser can paint immediately and load the rest of the styles afterwards. It is the usual fix for the “eliminate render-blocking resources” warning in PageSpeed Insights, and it improves Largest Contentful Paint, one of the Core Web Vitals Google measures.

By default a script tag stops the browser: it downloads the file, runs it, and only then carries on reading your page. A theme with half a dozen scripts in the head can hold the first paint for a second or more.

Deferring tells the browser to keep building the page and run the scripts once it has finished. Nothing is removed and nothing loads later than it should, it just stops blocking. DietPress handles the dependency order for you and lets you exclude any script that misbehaves, from the settings or with a filter.

Two reasons. Speed: a font from fonts.gstatic.com needs a fresh DNS lookup, TCP connection and TLS handshake to a domain the browser has never contacted, and browser cache partitioning means the visitor gets no benefit from having downloaded that font on another site. Privacy: serving them from Google transfers your visitor’s IP address to Google, which German and other European courts have ruled a GDPR violation.

DietPress downloads the fonts your theme uses to your own uploads folder and rewrites the stylesheets to point there. If anything fails it quietly falls back to Google, so a font never goes missing.

Plugins tend to load their CSS and JavaScript on every page of your site, whether or not the page uses them. A contact form plugin loads its scripts on every blog post; a slider plugin loads its libraries on pages with no slider; WooCommerce loads its cart on your About page.

DietPress detects which pages actually use each one and removes the rest, for WooCommerce, Contact Form 7, the block library, Slider Revolution, TablePress, Smash Balloon, Formidable Forms and Everest Forms. Pages built with a bui