I spent a while stacking caching plugins and wondering why things weren't as fast as they should be. The thing that fixed it wasn't a plugin, it was understanding that the three caching layers do completely different jobs and you build them from the bottom up.

OPcache is the foundation. It caches compiled PHP bytecode so the interpreter isn't recompiling your code on every request. It helps every single PHP app, it operates independently of everything else, and you basically never turn it off. In production I set validate_timestamps=0 and just flush it on deploy.

Object caching (Redis or Memcached) sits above that. It stores the results of expensive database queries so WordPress isn't making dozens of DB round trips per page. This is the layer that matters most for logged-in users and anything dynamic, because those requests skip page caching entirely.

Page caching (Nginx FastCGI cache for me) is the big hitter for anonymous traffic. A cached page never touches PHP or MySQL, it just serves HTML. Massive for traffic spikes, but useless for logged-in users, so you set proper bypass rules for wp-admin, carts, checkout and logged-in cookies.

The lightbulb moment was realizing they don't compete, they cover for each other. Page cache handles anonymous hits, object cache carries the cache misses and logged-in users, OPcache speeds up all the PHP underneath both. The mistakes I'd been making were running two page cache solutions at once and expecting object cache to help on pages that were already fully page-cached (it doesn't, page cache bypasses WordPress completely).

The other thing that clicked: the right strategy depends on your site. Marketing sites lean hard on page caching. Membership and ecommerce lean on object caching because everyone's logged in. Frequently updated content sites need shorter TTLs and smart invalidation.

I wrote the full guide up with the actual php.ini, wp-config and Nginx config snippets in this article Understanding WordPress Caching Layers: A Developer's Configuration Guide. Curious how you guys handle cache invalidation on fast-moving sites, that's the part I still tweak most.