While this is an impressive technical achievement, it's worth taking a step back to ask why IAM needs to serve 400M API calls every second when AWS has between one and two million active users. How would this number change if IAM were less complex?

Edit: I understand how every machine needs to invoke IAM APIs and how temporary credentials and other uses increase the number super-linearly with every active user. Still, 400M RPS (nearly 35B requests/day) could be reduced significantly by improving the underlying object model so it scales down better. Right now, even a simple Lambda function that needs to access other AWS resources requires 3 API calls: create a policy, create a role, and connect the two.