First, according to the Device support matrix
this is an unsupported configuration, DP 6.10 only supports up to LTO-4. I am not even sure if LTO-6 is supported on the latest version of DP
Having said that, I would look at concurrency as being a posible cause for this. Concurrency is the number of data streams that can be written to a media at the same time
Modern tape drives are capable of writing much faster than most data gets sent to them, so, as a result, there is sometimes a point where the tape drive stops, and waits for the Tape buffer to fill. When it is sufficiently filled, teh tpe has to re-position to the last file mark with some buffer characters, and starts writing to the tape again until, again, the tape buffer empties
To avoid this, you increase the concurrency to 10 -> 15, or even more. This is done in the Propertis of teh tape drive in the Backup specification -> Destination tab
Of course, you have to have enough data stremas to make use of the concurrency. A data stream is an Object (mounted File System), so, if you are only backing up 3 objects, you will never meet the concurrency value, and your backup will continue to be slow
I am going to try and attempt to attach a powerpoint presentation that illustrates this