Well, I can't reduce the size of the file : it's a full size backup and I need it as it is.
Then, as I told you, downloading the same file with another FTP client is much faster then with SmartFTP.
SmartFTP speed : about 80 Kb/s, duration : 16 hours
Other FTP client speed : about 170 Kb/s, duration : 8 hours.
I've not used both at the same time, but one after the other.
Is it possible to download and use an older version of SmartFTP ?