I've noticed that many people who send me their .INI files when asking for help/support with their issues, include "Priority=Idle" line.
In most cases you SHOULD NOT set Priority parameter, as normal priority will work best in most cases. Set this parameter only if your source and destination is file system and only if you are sure setting it will give you perfomance benefit.
If source or destination is FTP server, downloading/uploading to FTP server is very slow comparing to other processes, so FTPSync gives other processes enough time, regardles of this parameter. Also, setting "Priority=Idle", when your source or destination is FTP server may cause you frequent timeouts, when your computer is too busy with other processes.