Is your DLT connected directly to the server via scsi? What speed is the scsi? You said the files are coming down over the network – what is the speed of the nic?
One of these factors is slowing you down. If the DLT is connected directly to the server where the files are to be backed up from what is your throughput there?
Another thing to look at it what’s the disk queueing look like on the source drive? If the source drive can’t handle the data transfer speed then the backups will slow down.
I would also suggest checking your anti-virus. Maybe something has changed with it like a whole-disk scan during the backup period.