WordPress 6.1 introduces a number of important performance improvements which will have impact in all aspects. The most significant improvements are done in caching WP_Query as well as REST API.

Improvements to WP_Query

Database queries in WP_Query being cached is a long wanted feature for many developers and finally a dream come true in WordPress 6.1. A couple of new functions will ensure that users cache and linked objects for menu items are now primed while get_page_by_title function will from now on use WP_Query and take the full advantage of all these improvements.

Improvements to WP_Query performance in 6.1

Trac tickets holding these changes: #22176, #55716, #55620, #36905.

Improvements to REST API

Priming caches in a single query will significantly improve Posts controller with several new helper functions but that’s not all. User and comments controller will benefit in a similar way while taxonomy and post type controllers are now going to prepare_links only when needed. This control over prepare_links method will be available for custom controllers as well.

Performance improvements to the REST API

Trac tickets holding these changes: #52992, #56019, #56020, #55592, #55593, #55620, #55674, #56272.

Site Health improvements

WordPress 6.1 will have two new Site Health checks – Persistent Object Cache and Page Cache, along with a number of new filters. Find out more in dedicated Dev note.

New cache Site Health checks in WordPress 6.1

Trac tickets holding these changes: #56040, #56041.

Improvements to multisite

In WordPress 6.1 the network options functions have been updated to be consistent with other metadata types, support for register_meta is added, prime caching network options in a single query, string main site ID in network options and more.

Multisite improvements in WordPress 6.1

Trac tickets holding these changes: #37181, #55802.

Bootstrap/Load

The order of action hooks is slightly changed in order to gain more control over caching behavior, preloading assets and conditionally executing redirects.

Moving the send_headers action to later in the load

Trac ticket holding these changes: #56068.

Improvements to Cache API

Cache API improvements bring validating cache keys in WP_Object_Cache methods, deleting cache by group and making a number of private cache priming functions public, and thus, available for usage in plugins and themes.

A valid cache key must be either an integer number or a non-empty string. This will prevent silent failing in wp_cache_*() functions, done by a quick type check and adding a _doing_it_wrong() message if the string is empty, false, or null. Also, a check in update_user_caches() and clean_user_cache() will make sure that the email is not empty before being cached or removed from cache. Read more in #56198.

Several private cache priming functions for various object types are now public and plugin and theme authors are encouraged to use these functions to improve the performance of their code by reducing the number of database queries. These functions are:

• _prime_post_caches()
• _prime_term_caches()
• _prime_comment_caches()
• _prime_network_caches()
• _prime_site_caches()
• _get_non_cached_ids()

Read more in #56386.

Introducing wp_cache_flush_group() function

If you wanted to delete cached item with wp_cache_delete you had to specify the item’s ID, or you had to flush the entire cache with wp_cache_flush. WordPress 6.1 introduces a new plugable function called wp_cache_flush_group which removes all cache items in a group, if the object cache implementation supports it.

Introducing wp_cache_supports() function

Developers can now detect if their current implementation of an object cache supports flushing by group, by calling wp_cache_supports( $feature ) which returns true if the feature is supported. Third-party object cache plugins can declare a wp_cache_supports() function and correctly list their supported features:

• add_multiple
• set_multiple
• get_multiple
• delete_multiple
• flush_runtime
• flush_group

Note: The wp_cache_supports() function replaces and supersedes the wp_cache_supports_group_flush() function added in #4476.

Read more in #56605.

Media improvements

WordPress 6.1 will add decoding="async" to image attributes, along with new wp_img_tag_add_decoding_attr() function and wp_img_tag_add_decoding_attr filter. Read more in #53232.

Query-attachments AJAX endpoint now caches the parent post objects. Read more in #56037.

WP_Media_List_Table class will call update_post_parent_caches function in order to prime parent caches in a single database request. Read more in #56036.

Added caching to wp_count_attachments() for better consistency with wp_count_posts(). Read more in #55227.

Avoid duplicated query when retrieving empty posts collections. Read more in #55677.

Post, Post Types improvements

In WordPress 6.1 WP_Posts_List_Table class will call update_post_author_caches function in order to prime post author caches in a single database request. Read more in #56100.

A new filter post_class_taxonomies will allow developers to reduce the number of taxonomies for which classes term classes are generated. Read more in #37114.

Sites running persistent object caching will have result of database queries in _find_post_by_old_slug and _find_post_by_old_date functions, cached. Read more in #36723.

Editor

Additional build task copy:block-json will convert and store all block.json files in a single blocks-json.php which will prevent all of this from happening for every block on each page load.

Improved PHP performance for core blocks registration

Trac ticket holding these changes: #55005.

Database

Identifiers (such as Table and Field names) are escaped with the %i placeholder which will prevent SQL Injection Vulnerabilities and provide a small performance improvement.

Escaping Table and Field names with wpdb::prepare() in WordPress 6.1

Trac ticket holding these changes: #52506.

Other performance improvements

Administration

Performance of WP_List_Table::get_column_info() is improved by adding the primary column to the cached header values (#34564).

Cron API

In 6.1 wp-cron will be non-blocking for LiteSpeed LSAPI (#54668).

Taxonomy

Retain default term option when unregistering taxonomies and adjustments to handling default terms for custom taxonomies (#54472).

Themes

Two new actions are wrapping the process of loading a template file (load_template) – wp_before_load_template and wp_after_load_template (#54541).

Script loader

New filter wp_preload_resources enables resource preloading with rel='preload' (#42438).

Users

Prime user meta in WP_User_Query (#55594).

Thanks to @spacedmonkey, @tweetythierry, and @tillkruess for peer review.

#6-1, #dev-notes, #dev-notes-6-1, #field-guide, #performance