The Duplicate D2T is taking 10 x time of a Backup to tape B2T

Hello,

I am using BE2010R2 on WinSrv2008 to do B2D2T jobs. I noticet that the writing bitrate to tape is too small, about 400 Mb/min. After reading some discussions about this, decide to run a backup job to tape using the same data (files) used for duplicate before. Amaizing but the bitrate raised up to 4500 Mb/min. 

My qiestion is: why is it working like this and if possible how can i get the same speed for duplicate job on tape (I know that priviously, about 6 months ago the speed of the duplciate jobs was about 4000Mb/s and then start decreasing till now 400Mb/s)?

Thanks for any help.