Amazon S3 Bucket Keys reduce the costs of Server-Side Encryption with AWS Key Management Service (SSE-KMS)

Amazon S3 Bucket Keys reduce the request costs of Amazon S3 server-side encryption (SSE) with AWS Key Management Service (KMS) by up to 99% by decreasing the request traffic from S3 to KMS. With a few clicks in AWS Management Console and no changes to your client applications, you can configure your buckets to use an S3 Bucket Key for KMS-based encryption on new objects.  

Source:: Amazon AWS