Ftp – Windows Server 2008 with FileZilla, slow

First post on this site. Seems like a nice place. ^__^

Well, installed Windows Server 2008 as our dev server.
IIS works as it should, the remote desktop with TightVNC works but lags
and I need to refresh everything all the time.
But the real issue is with our ftp.

We're trying to use FileZilla. Connecting locally works great but when I try to connect from the same place using the external ip everything goes slow.

I can connect so the port must be open correctly but I can't do anything useful.
When I open a folder to check what's inside it takes one or two minutes to get the folder open.

Any ideas why it's like this?

Cheers,
nandarya.