You may be interested in the latest news re: allocating ENIs in VPCs for Lambda[1]. They've finally got the cold-start situation a little more under control in that scenario.
Here's the meat, to save you a click:
> AWS says that the one-time setup of the shared network interface can take 90 seconds to complete. But because the network interface gets created when Lambda functions are first created or VPC settings updated, the connectivity-related latency during cold start approaches zero. The same goes for function scaling, which no longer requires new network interfaces for each new execution environment.
Here's the meat, to save you a click:
> AWS says that the one-time setup of the shared network interface can take 90 seconds to complete. But because the network interface gets created when Lambda functions are first created or VPC settings updated, the connectivity-related latency during cold start approaches zero. The same goes for function scaling, which no longer requires new network interfaces for each new execution environment.
[1]: https://www.infoq.com/news/2019/09/aws-lambda-vpc/