The first cold start happens when the very first request comes in.

After that request is processed, the instance stays alive for the time being to be reused for subsequent requests. But for how long?

The following chart shows the typical range of cold starts in AWS Lambda, broken down per language. The darker ranges are the most common 67% of durations, and lighter ranges include 95%.

AWS Lambda has a setting to define the memory size that gets allocated to a single instance of a function. The CPU resources are allocated proportionally to the memory. So, in theory, larger instances could start faster.

AWS Lambda might need to access resources inside Amazon Virtual Private Cloud (Amazon VPC). In the past, configuring VPC access slowed down the cold starts significantly.