@Soren: I don't think you do anything wrong. It is the same behavior at my site with 24TB total size. (I thought it would be my problem only, as I could not find any other thread about it ...)
G'day Soren,
Sometimes this happens to us as well, I had put it down to problems with the Adaptec Raid controller, as we had no issues with the DSS Lite running on a PC Motherboard and software raid.
It seems to be mainly worse when the controller is rebuilding.... But that is only my untested gut feeling. Since the other services (iSCSI etc) work fine, we have just lived with it.
I too thought it was just our problem, didn't realise it was common.....