It it high load times on the server which is concerning your, or slow response on the client?
What is the API client? Another web app? A mobile app?
How frequently is the API client making requests?
You want to make sure that the server is set up correctly - slow response times for API calls is likely an indicator that your database is struggling. Make sure it is optimised and you've given it enough innodb_buffer_pool_size
(but not so much that you cause swapping). Is your MySQL server running on HDD or SSD?
At the client end - if the clients are making frequent requests for this data, ensure you are caching the responses on the clients so that they aren't continually making requests, causing higher load on the server.