When using WordPress REST API each request is processed by WordPress and depending on the size of the database and server performance, issues start occurring for high-traffic websites. By using a plugin such as WP REST Cache you can cache the responses of the REST API, making it much faster.
Then if you want to always view uncached REST API responses you can add the following line to the request header:
Cache-Control: private or Cache-Control: no-cache
Apart from caching the response, there are also a few more methods to speed up WP REST API response time, here are some useful guides:
- WP Rest API too slow? – Yair Levy @medium
- WordPress REST API Slow Response time – WP Q&A
- WP REST API vs admin-ajax.php vs Must-Use Plugin: Handling AJAX Requests in WordPress
- Speeding Up WordPress REST API Response Times with Cached Data