JSON:API in Drupal 8.8 is up to 3x faster than Drupal 8.7
Drupal 8.8.0 will be released in December 2019 and the upcoming changes in JSON:API module codebase introduce huge performance benefits.
jsonapi
How to create and expose computed properties to the REST API in Drupal 8
In Drupal 8.5.0, the "processed" property of text fields is available in REST which means that REST apps can render the HTML output of a textarea without worrying about the filter formats.
In this post, I will show you how you can add your own processed fields to be output via the REST API.