We Cut Our Lambda Bill by 33% Switching to Rust — And AI Wrote the Code

We ran 50,000 event registration records through identical Lambda functions — one in Python, one in Rust — processing CSV and JSON files from S3 into DynamoDB. Rust completed the work in 12-15 seconds. Python took 20-21 seconds. Same memory (1024MB), same architecture (ARM64/Graviton), same data, same DynamoDB tables.