If you were trying to use Lambda in a use case that was very latency sensitive, cold starts were probably your greatest concern. AWS has heard the concerns

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AWS has launched Provisioned Concurrency as a feature for Lambda, which allows users to configure warm instances and solve the cold start issue without code changes. Provisioned Concurrency can be set in the AWS Console or via the Serverless Framework, and it can also be scheduled for specific time periods.

Provisioned Concurrency: What it is and how to use it with the Serverless Framework