“Install a caching plugin” is standard advice and it’s incomplete. WordPress gets faster from three distinct layers, and each skips a different amount of work. Adding all three without understanding the split is how you end up with a site that’s fast for logged-out visitors and unusable in the admin.
Layer one: the page cache
A page cache stores finished HTML for a URL and serves it without booting PHP or querying the database. On a typical WordPress site that removes something like 90% of the work per request.
What it skips: PHP execution, every database query, all theme and plugin code.
What it can’t skip: logged-in users, carts, anything personalized — unless you vary the cache on cookies carefully. Which is why “it’s fast in incognito” is not a performance test.
Layer two: the object cache
WordPress stores query results and options in memory as objects. By default that memory dies with the request, so a busy page rebuilds the same objects every load. A persistent object cache — Redis or Memcached — keeps them between requests.
What it skips: repeated database queries for options, meta, and term lookups.
Where it earns its keep: WooCommerce, memberships, heavy admin use. On a simple blog with a working page cache the gain is real but modest.
The cost: one more daemon, 30–80 MB of RAM, and a new way to fail. If Redis dies, WordPress should fall back to the database gracefully. Verify that before you rely on it.
Layer three: the CDN
A CDN terminates requests at an edge location near the visitor. For static assets that’s straightforward. For HTML it can also cache the page itself, cutting latency and origin load at once.
What it skips: geographic round trips, TLS handshakes to your server, and traffic reaching your box at all.
What it adds: invalidation complexity. Purge incorrectly and visitors get stale pages. Most CDN problems are invalidation problems, not performance problems.
Which layer are you missing?
| Symptom | Likely layer | First thing to try |
|---|---|---|
| TTFB over 300ms on repeat visits | Page cache | Confirm caching is on for that URL and no exclude rule swallows it |
| Admin and checkout slow, front end fine | Object cache | Enable Redis and watch the hit rate |
| Fast locally, slow for overseas visitors | CDN | Check audience geography in analytics before paying for one |
| Fast everywhere but Core Web Vitals still poor | None of these | It’s images, fonts, or render-blocking scripts |
| High hit rate but pages are wrong | Invalidation | Exclude cookies, carts and logged-in users properly |
The order to add them
- Page cache. Biggest return for least effort. Always first.
- CDN. When you have visitors far from your server region, or want a buffer against spikes.
- Object cache. When a page cache exists and dynamic or admin work is still slow.
Putting an object cache on a site with no page cache is the common misstep: you speed up database queries while still paying for full PHP execution on every hit.
What caching never fixes
Heavy plugins, an oversized media library, unoptimized images, render-blocking third-party scripts. A page cache makes a slow site look fine to a crawler and keeps it slow for the humans who matter — the ones who leave because the hero image is 3 MB. Convert to WebP, serve correctly sized files, audit the plugin list before blaming the host.
Questions people ask
Do I need a caching plugin if my host already caches at the server level?
Probably not a second one. Two stacked page caches produce stale content and confusing debug sessions. Find out what your host runs, then add only what’s missing.
Is Redis worth it on a 2 GB VPS?
Yes if the site is dynamic — WooCommerce, memberships, heavy admin. On a brochure site with a page cache, spend the 60 MB elsewhere.
Does a CDN help SEO?
Indirectly, through Core Web Vitals and availability. It doesn’t move rankings on its own. Speed helps; a CDN is one route to it.
Sources
- https://developer.wordpress.org/advanced-administration/performance/cache/
- https://web.dev/articles/ttfb
- https://redis.io/docs/latest/operate/oss_and_stack/management/optimization/memory-optimization/
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