By default, Function Compute runs functions on on-demand instances. These instances are created automatically when a request arrives and are destroyed after sitting idle for usually 3 to 5 minutes. When a request hits a function with no running instance, Function Compute must initialize a new execution environment before running the function — this process is called a cold start.
Infrequently invoked functions experience cold starts on nearly every invocation, because the previous instance is destroyed before the next request arrives.