Extremely poor SFTP speed (35KB/s)?

On both Nexus 7 (2012) and Samsung Galaxy S3, SFTP to my Mac using ES File Explorer reaches only 35KB/s. On AndFTP, I get around 200KB/s. How come the speed is so bad? If I FTP to the same machine from the same devices, I get over 1.3MB/s on ES. How come SFTP speed is so terrible?

Thanks