Lambda invocations after idle periods (>5-15 min) or during scale-up take significantly longer (originally 10-20s, now 1-5s+) than warm invocations (<500ms). CloudWatch Logs show high Init Duration in REPORT lines, e.g., \"Init Duration: 10000.00 ms\". First invocation post-deployment or config change especially slow.

When a Lambda function is configured to access VPC resources, AWS must provision and attach an Elastic Network Interface (ENI) to the execution environment during cold starts. Prior to 2019 Hyperplane improvements, this added 10+ seconds; post-improvement, ENIs are pre-provisioned per subnet/security group combo and tunneled, but new concurrent executions beyond warmed capacity still incur ENI attachment/setup overhead (now ~1s vs 10s+), plus standard init (code download/runtime).