SFTP slow performance - intermittent issue

We are using the WINSCP for SFTP file transfer. SFTP command triggering from informatica application server.
Sometimes it is completing the 40 MB file transfer less than a minute. But sometimes it is taking couple of minutes to complete.

Our observation. When we see the below message "Increasing the send buffer value from current value to double, it will transfer the file less than a minute.
When no change in Increasing send buffer value . It is taking long time to transfer the files.
Please suggest us do we have any parameters to change in WINSCP/SFTP client.

Please find below log for reference.
• Tried to compare SFTP logs for good performance (26 secs) vs. poor performance (12 mins 30 secs). In both the cases file size remains same.
• During slow performance, I don’t see Increasing send buffer entries and ~7 mins between below two events (in attached file, refer log events between 2021-03-03 02:41:04.955 and 2021-03-03 02:53:30.483)
o Initially buffer size is set to 262144 and later it’s not increasing
• During good performance, I see entries with Increasing send buffer. Not really sure how this is controlled and whether this is cause for poor performance (in log file, refer log events between 2021-03-03 03:39:07.966 and 2021-03-03 03:39:30.203)
o Initially buffer size is set to 262144
o Later it’s increased to 2097152

The increase of the send buffer comes in a response to SIO_IDEAL_SEND_BACKLOG_QUERY:
https://learn.microsoft.com/en-us/windows/win32/winsock/sio-ideal-send-backlog-query
How that query determines, if the buffer should be increased, is out of WinSCP control.

Though you can fix the buffer to large initial value using SendBuf raw session settings:
https://winscp.net/eng/docs/rawsettings#sendbuf

Hi Martin,

Thanks for the update. We have tried to update the send Buffer value , SshSimple.

SendBuf Optimize connection buffer size 0 = Off, Any positive value = On, value indicates size of the buffer, with recommended value of 262144
The SshSimple may need to be set to 1 to disable some of the optimizations, that unchecking the GUI option disables.

We have updated the values as below But after saving these values are disappear automatically. It is not saving permanently. Could you please suggest to save the settings permanently
Regards,
Srinivas

I didn't ask you to set SshSimple. When exactly does the SendBuf disappear automatically? It does not disappear for me.