Article summarizing recent performance optimization work on a secondary site of the Encyclopedia of World Problems and Human Potential: https://www.vacilando.org/article/using-cdn-accelerate-drupal-api-sourcing-gatsbyjs
The GatsbyJS public sandbox site at www.askthefox.org has always been very fast, but its build time was growing agonizingly long. That was caused primarily by the amount of data and frequency of requests to the Drupal backend (= the EWPHP editing site).
To solve the problem we have employed a modern Content Delivery Network (CDN) in combination with a successful code improvement in the core of GatsbyJS.