Give it some buffers, man... 
copy dh0: to cf0: all quiet clone buffer=1024
The QUIET option certainly speeds things up too.
Like Thomas said though, the OP's issue sounds a lot like incorrect MaxTransfer setting.
hrmmmm never bothered with buffers, other than the filesystem buffers. I suppose that would help, eh? As for quiet, it's funny, I always tar/gtar quiet, but when I do a copy like this I have a control complex which make me want to see the progress.
MaxTransfer is a funny animal, too. When I installed an SSD on my CS MKIII with SCSI-to-SATA, HDToolBox set the MaxTransfer to 0x0001FE00. I suspect I can safely change it but I haven't bothered while everything works.
