What each layer does
- Page cache: stores the finished HTML so WordPress does not rebuild it
- Object cache: stores database query results
- Browser cache: tells returning visitors to reuse files they already have
- CDN: serves static files from somewhere near the visitor
Page caching gives the largest gain by a wide margin on most sites.
Where it goes wrong
- Changes not showing — you are seeing the cached version
- Logged-in users seeing cached pages meant for visitors
- Carts and checkouts cached, which is genuinely damaging
- Forms failing because a security token was cached
Caching a shop's cart page is the classic disaster: customers see each other's baskets. Exclusion rules for cart, checkout and account pages are not optional.
Configure, do not just install
A caching plugin installed and left at defaults does a fraction of what it could. Exclusions, expiry times and what to preload all need setting for your site.
Fifteen minutes of configuration is worth more than the plugin choice.
Clearing it properly
- Cache cleared automatically when content is published or updated
- Someone knows how to clear it manually
- Editors told that changes may take a moment to appear
- Cleared after any plugin or theme change
Server-level beats plugin-level
Caching at the server, as good managed hosts provide, is faster and more reliable than a plugin doing the same job in PHP.
If your host offers it, use it and skip the plugin. One less thing to maintain.