S3的Golang AWS SDK是否受到限制？

I'm using go aws sdk to put object to s3. It takes about 1 second to upload the file. To test the concurrency, I have an http server that I hit with ab -c10 -n10 http://localhost:8080 and the handler calls this code:

func (service *Service) LogResponseManager(resp gorequest.Response, s3key string) {
    uploader := service.S3Manager
    defer resp.Body.Close()
    params := &s3manager.UploadInput{
        Bucket:      aws.String("cslogs.sellbrite.com"), // Required
        Key:         aws.String(s3key),                  // Required
        Body:        resp.Body,
        ContentType: aws.String("text/plain"),
    }
    uploader.Upload(params)

}

I'm using the echo server to receive requests and if I were to just sleep 1 second in the handler the time to execute all 10 requests in parallel is just 1 second. However when uploading too s3, the time to execute 10 concurrent requests seems to be adding up in serial, close to 8 seconds. Is the go sdk not executing s3 uploads concurrently?