When I discuss AWS Lambda cold starts with folks in the context of API Gateway, I often get responses along the line of:
Meh, it’s only the first request right? So what if one request is slow, the next million requests would be fast.
Unfortunately that is an oversimplification of what happens.
Cold start happens once for each concurrent execution of your function.
API Gateway would reuse concurrent executions of your function that are already running if possible, and based on my observations, might even queue up requests for a short time in hope that one of the concurrent executions would finish and can be reused.