This article is part of a series on WordPress Optimization.

WordPress caching is the fastest way to improve performance. If your site is getting hit right now install W3 Total Cache, WP Super Cache or Cache Enabler.

Caching Plugins

Plugins like W3 Total Cache, WP Super Cache and Cache Enabler can be easily installed and will cache your WordPress posts and pages as static files. These static files are then served to users, reducing the processing load on the server. This can improve performance several hundred times over for fairly static pages.

When combined with a system level page cache such as Varnish, this can be quite powerful.

If your posts/pages have a lot of dynamic content configuring caching can be more complex. Search for “WordPress cache plugin” for more info.

Browser Caching

Browser caching can help to reduce server load by reducing the number of requests per page. For example, by setting the correct file headers on files that don’t change (static files like images, CSS, JavaScript etc) browsers will then cache these files on your visitor’s computer. This technique allows the browser to check to see if files have changed, instead of simply requesting them. The result is your web server can answer many more 304 responses, confirming that a file is unchanged, instead of 200 responses, which require the file to be sent.

Look into HTTP Cache-Control (specifically max-age) and Expires headers, as well as Entity Tags for more information.

Object Caching

Object caching in WordPress is the act of moving data from a place of expensive and slow retrieval to a place of cheap and fast retrieval. An object cache is also typically persistent, meaning that data cached during one request is available during subsequent requests.

In addition to making data access much easier, cached data should always be replaceable and regenerable. If an application experiences database corruption (e.g., MySQL, Postgres, Couchbase), there will and should be severe consequences for this database (and let us hope that there is a good backup plan in place). In contrast with the main data store for the application, if a cache is corrupted, the application should continue to function as the cached data should regenerate itself. No data will be lost, although there will likely be some performance problems as the cache regenerates.

The storage engine for an object cache can be a number of technologies. Popular object caching engines include Redis, Memcached, APC, and the file system. The caching engine used should be dictated by the needs of the application. Each has its advantages and disadvantages. At a bare minimum the engine used should make accessing the data more performant than regenerating the data.

Server Caching

Web server caching is more complex but is used in very high traffic sites. A wide range of options are available, beyond the scope of this article. The simplest solutions start with the server caching locally while more complex and involved systems may use multiple caching servers (also known as reverse proxy servers) “in front” of web servers where the WordPress application is actually running.

Adding an opcode cache like Opcache, or WinCache on IIS, to your server will improve PHP’s performance by many times.

Varnish cache is very powerful when used with a WordPress caching plugin such as W3TC.

Further Reading

• Core Caching Concepts in WordPress
• Best Practices for Speeding Up Your Web Site – Expires / Cache-Control Header and ETags (by Yahoo! Developer Network)
• WebSiteOptimization.com: Use Server Cache Control to Improve Performance