This is the kind of behaviour that would be expected if Alt.Binz can't find articles on the server, and is waiting to retry them before completing the yEnc decoding - but your Log should tell you if this is the case. See settings in Setup > Connection.
Needless to say, you don't want to be running any other apps in the Alt.Binz directories (download & temp), and if you're running more than one client on the same server you'll need to make sure there are enough connection threads available for both.