WordPress 6.1 brings a number of key improvements to the REST API to increase performance. These improvements decrease the number of database queries that are run on each REST API request. 

Avoid unnecessarily preparing item links

Prior to WordPress 6.1, the prepare_links method in the REST API was called in all controllers. If the _fields parameter is passed to the REST API request, it might mean that the links field is not requested and would never be returned in the response. This is wasteful, as prepare_links can contain database calls or other complex logic that would be run even if it never returned the response. 

In 6.1 prepare_links are only called if requested in the response, when links are requested in fields or the _embedded parameter is passed. As part of this work, the taxonomy and post type controllers have now been updated to implement said prepare_links method to bring them in line with other REST API controllers. 

This is the code example of implementing this change in custom REST API controllers.

Before

$response = rest_ensure_response( $data );
$links = $this->prepare_links( $post );
$response->add_links( $links );

return $response;

After

$response = rest_ensure_response( $data );

if ( rest_is_field_included( '_links', $fields ) || rest_is_field_included( '_embedded', $fields ) ) {
  $links = $this->prepare_links( $post );
  $response->add_links( $links );
}

return $response;

This logic conditionally calls prepare_links only if _links or _embedded is requested. 

For more info see Trac ticket: #52992, #56019, #56020

Improvement to the Posts controller

When running profiling tools against the responses of REST API requests, it was discovered that post controllers request a lot of linked data to each post. For example, when returning a post in a REST API response, linked data such as author (user), featured image, and parent post were all requested. As these linked items were not primed in caches, it could mean that for each post in the REST API response there would be 3 separate database queries: one for the user, one for the featured image, and another for the parent post. 

In WordPress 6.1 all the caches are primed in a single database query and there are new helper functions to enable this:

update_post_author_caches

Takes an array of posts and primes users caches in a single query. 

update_post_parent_caches

Takes an array of posts and primes post parents in a single query. 

update_menu_item_cache

Takes an array of posts and primes post / terms link to menu items single query. 

The existing function update_post_thumbnail_cache was used to prime featured image caches. These functions are also being rolled out to other parts of the core that can benefit from priming caches in a single place. 

For more info see Trac tickets: #55592, #55593, #55620    

Improvements to other controllers

The comments and user controllers have also been improved: User controller now primes user meta in a single query and the comments controller now primes the linked post cache in a single query. Improvements were made to the post search controller to improve database performance along with the media controller. 

For more info see Trac tickets: #55674, #56272, #55677, #55716

Thanks to @flixos90 and @milana_cap for peer review and @shetheliving for proofreading.

#6-1, #core-restapi, #dev-notes, #dev-notes-6-1, #performance, #rest-api