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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.