I transfer a Single file using WinSCP FTP on FTP server.
(No other load on FTP server, only single connection)

File size 1.27GB, and It takes approx. 22 secs.

But when I transfer same file using WinSCP SFTP on SFTP server.
(No other load on SFTP server, only single connection)

File Size 1.27GB, and it takes aarox. 15 minutes or 900 secs.
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My client PC Configuration are:

32-Bit OS,
RAM - 4GB,
Processor - Intel(R) Core (TM) 2 Duo E7500 @ 2.93 GHz
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Server Configuration are:

MS Windows 2003 R2 with SP2,
RAM - 2 GB,
Processor - Intel(R) Xeon (R) CPU E5320 @ 1.86 GHz
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Network BandWidth is 100 MBps
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Please Suggest How to improve the SFTP performance?