API testing is a type of software testing where application programming interfaces (APIs) are tested to determine whether they meet expectations for functionality, reliability, performance, and security. In simple terms, the purpose of API testing is to reveal bugs, anomalies, or deviations from the expected behavior of the API. 

API testing does not add to the look and feel of the software product. As an alternative to using standard input devices (such as a keyboard) or output devices (such as a monitor), API testing uses software to send calls to your API that you use to receive output during API testing and ultimately to your will do to document the response product.

Spend some time looking through your site’s speed test results and look for the performance that has the greatest impact on user experience. Focus on those high-impact factors and take the necessary steps to get them into shape.