Why is the upload speed of dmp files from aws rds to s3 so fast?

The time will vary depending on file size. There is a way to monitor upload download files, for example with RDS for Oracle you can run a query as described here: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/oracle-s3-integration.html

SELECT rdsadmin.rdsadmin_s3_tasks.upload_to_s3(
      p_bucket_name               =>  's3bucketOwnedByAccountB', 
      p_prefix                    =>  '', 
      p_s3_prefix                 =>  '', 
      p_directory_name            =>  'DATA_PUMP_DIR',
      p_bucket_owner_full_control =>  'FULL_CONTROL',
      p_compression_level         =>  6) 
   AS TASK_ID FROM DUAL;

SELECT text FROM table(rdsadmin.rds_file_util.read_text_file('BDUMP','dbtask-task-id.log'));