On WordPress.com, options are cached in memory to avoid database lookups, which speed things up. This is only effective if the cached object is kept small. Once the object reaches a certain limit (1MB) it will no longer cache and requests are sent to the database servers, which, depending on the traffic of the site, can cause a flood of requests and have a severe impact on performance.
Here are several guidelines you should keep in mind when working with options as storage containers.
- Be sure the sum of all data you store in options (
get_option(), etc.) is less than 400 KB at most. Options are meant for settings and plugin options. If you need to store bigger HTML fragments, consider using a custom post type or the wp-large-options plugin.
- Each WP_Cache object can be no larger than 1MB or it will fail to cache. Where applicable and necessary, store items in individual cache objects rather than a single large cache object in order to avoid potentially hitting the limit.
- Always store only what is really necessary in options. For example: If you need only the post_id, title or link of a post, don’t store the full post object.
- Make sure your data is sanitized and validated.
- Check that the data is not escaped twice. Double escaping data, especially if it is automatically filled, can cause the values to grow out of control.
- Never write options from the front-end. Write operations caused by front-end functionality are generally a bad idea and will slow down your site.