1 Gbps LAN means that your LAN can (theoretical) transport 125 MB/s. The speed of LTO4 is 190 MB/s - so your LAN delivers only 65,79% of the speed of your tape drive.
A tape drive isn't able to wait for data - it has to write the data in...
which speed has your LAN Environment? Or: How dow you get the data from the VMWare host(s) to the Backup Exec Server?
Dedup is for saving space, not for speeding up backups :)
So, as pkh already mentioned, do a VMDK-Backup to disk: