I have been using coreftp server for more than 1 year, and have serious issues with sftp speed. I just tested several clients uploading a 1.7 gig ISO file over a fast broadband connection (1-2+ MB/s) using CoreFTP server version 3.13:
All clients installed on Vista SP2:
CoreFTP LE 2.1 b1625: ~89 KB/s, about 5 hours to transfer
Filezilla 3.2.8.1: ~118 KB/s, about 4.25 hours to transfer
WinSCP 1.9 ~114 KB/s, about 4.25 hours to transfer
Bitvise Tunnelier 4.30 ~350 KB/s, about 1.5 hours to transfer
Unsecure FTP to a similar server, ~500 KB/s+, about 2 hours to transfer
The only one worthy of using for anything other than small transfers is Bitvise. Unfortunately, it is not free except for personal use. Not good when you have users from all over the world that cannot be asked to pay for a client. I am considering giving up on sftp and moving to ftps, and dealing with all the firewall issues.