API latency, a critical factor in application performance, refers to the time it takes for a request to an API to receive a response, encompassing the duration from the request's initiation to the receipt of the first byte. High latency can lead to delays that affect usability, making it important to understand and mitigate its causes, which include network delays, server processing time, database performance, third-party dependencies, and lack of caching. Distinct from API response time—which measures the total time until the full response is received—latency specifically tracks time to the first byte. Measuring and reducing API latency involves various techniques such as caching, optimizing backend logic, reducing payload size, using distributed infrastructure, and tuning TLS/SSL settings. Maintaining low latency requires continuous monitoring, profiling, and adherence to best practices like smart caching and minimizing unnecessary work per request, ensuring applications remain fast and reliable, thereby enhancing user satisfaction and business metrics.