Caching Can’t Speed Up Everything

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Caching Can’t Speed Up Everything

Caching is a crucial part of almost any website’s performance plan. Good caching strategies can work at different levels to accelerate the user experience in a number of ways. But even with the most perfect caching in place, some parts of the WordPress experience gain nothing.

Like most contributors to WP performance, you need to be aware of where your efforts to add speed through caching will pay off—and where they will do nothing.

Caching is great for static content. Images can load faster and pages can be served up nice and quick. But the flipside of this is that dynamic content misses out. A lot of this time, this is where you really want your user experience to be at its best.

One big example is the ecommerce purchase journey. To put it simply, you can’t easily cache a shopping cart. Misapplied caching can even slow shoppers down. If customers enjoy browsing your products quickly, but then suffer a slow slog when they’re actually paying for things, you can expect abandonment rates to accelerate.

But it’s not just checkouts. Any time your site or code needs to talk to the server, there’s no caching involved. So caching doesn’t make the user experience any faster when it comes to:

  • External API calls. Unless you cache the response manually (e.g., with transients), every call still waits on the server.

  • Database writes. Submitting forms, leaving comments, and updating user data are examples that always hit the database directly—no cache involved.

  • Dynamic, per-user content. We’ve already mentioned shopping carts, but there’s more. Logged-in dashboards, personalized recommendations and anything else tailored to individual visitors can’t be cached without custom logic.

  • Plugins that bypass the cache. Some plugins run their own database queries, don’t integrate with object caching, use inline scripts, or use real-time data. These can add overhead even when caching is enabled.

  • Search and filter queries. In particular, advanced queries (especially in WooCommerce) can be slow unless explicitly optimized.

  • Cold cache scenarios. The unavoidable fact of caching is that first-time visits, or visits after a cache purge, run uncached.

Then there’s another side to what we call the “WordPress experience”. It’s not just website visitors or users who are slowed down by slow WP. It also affects you, the developer.

The WordPress admin area is typically uncached to preserve live functionality, meaning it still relies on raw database and PHP performance. Heavy background processing is likewise a cache-free zone. Tasks like image uploads, CSV imports, and video encoding are CPU-bound, so if you want to speed them up you need to look at PHP performance. That’s why we made PHP Vitals.

PHP Performance is a Whole-of-WordPress Contributor

One of the great things about focusing on PHP performance as a measure of WordPress performance is that it affects every aspect of the WordPress experience. PHP is involved in every page load, every user interaction, every call to the server, running every plugin, and even things that your site visitors never see, like the admin area.

That’s why PHP Vitals, which looks at PHP performance in dozens of different ways, is such a strong tool for comparing different WordPress environments.

If you pair a clever and careful approach to caching with hosting infrastructure that runs fast PHP, you’re well on your way to offering a WordPress experience that’s genuinely better than most. You can check the second part of the equation with PHP Vitals right now.