Using Lambda functions stored in ECR has no impact on performance from my experience. AWS Lambda uses Firecracker under the hood, which builds a VM from a container image. It's likely that non-ECR image based Lambdas are actually packaged as a container image before being launched into a Firecracker VM.

https://aws.amazon.com/blogs/aws/firecracker-lightweight-vir...

This has been my experience as well, there was little to no impact on execution time at least.