IMAP Migration Tools

Hi,

I am looking for tools to migrate Mails from one IMAP-Server to another. Until now I have always used imapsync, but it seems very slow. Even over a 2x10GB/s connection I only get speeds of about 600KiB/s, because the programm always pauses betweeen every single mail. Does anyone here have a suggestion for an alternative?

Thanks in advance Julian