You've told a little about your hardware, but not enough. The off and on writing would indicate that it was shoe-shining -- that is, if you can't drive it fast enough, it has to stop, backup, reposition, and prepare to continue writing. That drastically slows it down when it is already slow. If you can maintain the data throughput to keep it writing, then it will maintain its speed.
So, where is the slowup? What is the drive you are pulling from? What is the hardware in general? What else is the server doing? What if you tried just a dd, which wouldn't mess with any processing, but just shove data at it? Or try one of the tape testing utilities, such as amtapetype, which comes with amanda (see http://www.linuxquestions.org/bookmarks/tags/backup
). That will drive it hard and tell you what your setup can do.
Anytime you are dealing with driving LTO, you have to look at your entire system and the data path in terms of hardware paths and speeds.