Please post verbose log file both from WinSCP and FileZilla showing the problem.
File calculating for background transfer - most important minus of winscp.
For what this calculating ?
Winscp - calcualting 30 minutes, transfer 10 minutes.
Filezilla and all others - transfer 10 minutes.
Simple transfer and all. Why not ?
This is very simple for code i think. 1 day work.
No one FTP clients dont have this . Always i must use filezilla for copy big data.
But i like winscp.