cpu hike for aws operations

We are getting cpu hikes for any aws operation (like listBuckets(s3), listQueues(sqs) and putItem(dynamodb) ). What could be the reason for cpu hikes?

Our backend is Nodejs and aws library is aws-sdk

Example

for (var i = 0; i < 100; i++) {
        s3.client.listBuckets(function (err, data) {
         console.log(data);
         });

    }

For the above loop %cpu going to 25

My problem is that for just an api call cpu should not go to that much hike.

For the same loop we tested with redis api calls, for that we are getting just below 2% cpu usage. Why is that only for aws operations?