Very slow requests to dynamodb from lambda function

I have lambda function and dynamo db table in the same region (us-east-1). In lambda function I perform very simple query:

params =
  TableName: 'users'
  Item:
    email:
      S: event.body.email
  ConditionExpression: 'attribute_not_exists (email)'
dynamodb.putItem(params, context.done)

There are only few rows in DynamoDB table, there is Hash Key on email and Read/Write throughtputs are set to 5/5.

Lambda function exeutes in ~4 seconds... This is very slow. Am I doing something wrong?