Some Lambda functions perform synchronous calls to other services, APIs, or internal microservices and wait for the response before proceeding. During this time, the Lambda is idle from a compute perspective but still fully billed. This anti-pattern can lead to unnecessarily long durations and elevated costs, especially when repeated across high-volume workflows or under memory-intensive configurations.

While this behavior might be functionally correct, it is rarely optimal. Asynchronous invocation patterns—such as decoupling downstream calls with queues, events, or callbacks—can reduce runtime and avoid charging for waiting time. However, detecting this inefficiency is nontrivial, as high duration alone doesn’t always indicate synchronous waiting. Understanding function logic and workload patterns is key.