WordPress internal object cache killed this site’s performance

As you may have noticed, this blog became very sluggish recently. Fortunately, I’ve narrowed down the cause of the lag to the WordPress internal object cache, which is supposed to increase site performance, but in my case it slowed down page load times by more than 10x, sometimes much more.

My blog’s symptoms:

Disabling all plugins and reverting to the default theme and displaying only 1 blog post on the homepage did not really affect my page loadtime.
The load delay was not an issue with PHP/MySQL at all. My page was completely executed and outputted by PHP in about 0.5-1 second (according to various plugins that showed the script execution time), but it took 10x that to cache the entire page to disk or load 100% of the page in a browser.
The page load lag has nothing to do with the browser on the client side. I tried Firefox, IE, Safari, and even external tools like Pingdom Full Page Tester.
The browser loaded 95% of the page and for some reason stalled for a long time on the footer section; making the footer empty didn’t help, and profiling the footer itself showed that PHP executed it in 0.06 seconds.

On a whim, I removed the following line that I inserted in wp-config.php to get back to a vanilla WP setup.

define(ENABLE_CACHE, true);

This line enables WP’s built-in object cache and is supposed to improve performance slightly. Problem solved!

This object cache supposedly caches database queries to disk to reduce CPU and database overhead. This should increase performance in most cases unless your disk I/O performance is very poor, like Dreamhost’s NFS storage system. This feature seems to be undocumented and had security issues in the past, but is commonly recommended as a safe way of increasing performance. I have heard that the object cache hardly improves performance, but I’d never imagined that it would slow my site down by 10x or more.

Disabling the object cache brought my page load times back to normal, with all my plugins enabled. I suppose the feature is disabled by default for a reason. If your blog is suffering from performance issues and your plugins or themes are not the cause, try disabling the object cache (if you enabled it). Comment out

define(ENABLE_CACHE, true);

in wp-config.php if needed.

However, some of the performance improvements in the upcoming WordPress 2.4 apparently have to do with the object cache. /me confused. I will try to find out more about what exactly the object cache does and what it did to kill my site’s performance. Thanks again, Jeff from NFSN, for helping me troubleshoot my site’s performance issues.

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