I understand that you are experiencing poor performance over SFTP, please answer all of the following questions.
Are you using the latest version of FlashFXP? If not please download the latest version of
FlashFXP.
What version of Windows are you using?
Does this happen when uploading or downloading?
Are you using a software firewall, such as Windows Firewall, ESET Smart Security, McAfee, etc?
There are several factors that can have a major impact on transfer speed, such as the encryption algorithm, message authentication codes (MACs), and compression. When comparing your results with other clients its important to make sure that these values match across clients.
Also what brand of software is the SFTP and what is the version? Different software brands sometimes require special internal settings for optimal performance or compatibility. Sometimes these settings change and need to be revisited.
The post you referenced is quite old and while adjusting the TCP/IP Buffer size can improve performance in some cases, typically you'll only gain about around 5%. However when set too high or low it can reduce performance by up to 50%.