A “cold start” is the 1st request that a new Lambda worker handles. This request takes longer to process because the Lambda service needs to: find a space in its EC2 fleet to allocate the worker. initialize the worker.  

https://lumigo.io/blog/this-is-all-you-need-to-know-about-lambda-cold-starts/#:~:text=A%20%E2%80%9Ccold%20start%E2%80%9D%20is%20the,initialize%20the%20worker