Poor performance TL2000 LTO6 and iSCSI Bridge

Hi, I've got a serious performance problem with a TL2000 (LTO6) and an iSCSI Bridge.
Both the TL and the iSCSI Bridge are running the most recent firmware (at the date of March 29th 2016).

The backup server is running Windows 2008 R2 Sp1 with latest updates installed; the server is using the recommended driver for the TL.
The backup software used is Netvault 11.

The TL is connected to the bridge using the supplied SAS cable and the bridge is connected to the sever with a N4032 switch (latest firmware here too), using a separate vlan, jumbo MTU enabled (and on iSCSI Bridge and server's nics too).

I configured both network cards on the bridge, same subnet, different ip and jumbo mtu.

I used this guide to connect the server: www.dell.com/.../EN , while I followed the user's guide to configure the iSCSI Bridge.

The problem is that the backup performance is unacceptable: backups on tape run at 5 MBs, while the same backups but on disk run fine.

I tried with jumbo mtu on and off, with a single iscsi connection or multiple iscsi connections, using a single network card on the bridge and then both again on. I tried all of the driver versions available on Dell Support for TL2000. I restarted TL, iSCSi Bridge and the server too: backups on tape keep running incredibly slow.

IBM tool shows me both the library and the tape drive as OK.

If I use a single iSCSI connection the total throughput is about 5MBs, if I use two connections, the total throughput is always 5MBs, about 2.5MBs each connection.

I tried to disable TUR (even if is there only a single server connected to the TL2000), but after that Netvault lost the drive and in Windows Device Manager the drive did show a yellow exclamation mark, saying that the driver could not be initialized.

Any idea or suggestion about this problem?

Thank you in advance!