Middleware serves as a crucial design pattern in web API frameworks, facilitating cross-cutting concerns like logging, authentication, and response transformations by operating at specific stages in the HTTP pipeline. This pattern, known by various names depending on the framework—such as middleware in NodeJS and Rails, filters in Java Servlet, and delegate handlers in C#—enables developers to handle these concerns without cluttering the core business logic of controllers. Middleware can be both passive, like logging, or active, such as compressing response bodies, and its performance is critical since any latency introduced can affect the entire application. Non-blocking frameworks like NodeJS focus on improving bandwidth, while others may require asynchronous handling to manage latency effectively. The order in which middleware is executed is also important, impacting both performance and security, and developers must consider underlying frameworks when building middleware to maximize efficiency and functionality.