I performed a quick test using boxcryptor 184.108.40.206 to see how fast it really is when mounting point is on a secure network or in this case an sftp.
1- using a free SftpNetDrive http://www.eldos.com/sftp-net-drive/
2- a private sftp account with simple hosting plan on www.gandi.net
3- mounted drive K: to the sftp , created an encrypted box on K: and mounted on L:
so that is K: is sftpdrive , L: is boxcryptor folder
4- used a binary file size of 12674015 ~12MB
5- copied 1 time the file from local location to drive K: , took measurements.
6- copied 1 time the file from location to drive L:
to drive K: , Completed in 00:00:28.836, 429 KB/s
to drive L: , Completed in 00:03:55.742, 52 KB/s
as you can see the difference is huge, for the time being my advise :
use local folders "mount points" to encrypt files, then copy the result to the sftp or network drive.
i hope this helps someone.