Very slow rate on fileserver backup with Backup Exec 2012 SP 4

Hello,

I've read a lot of post here but didn't find solution. I'm using Backup Exec 2012 SP 4 to backup file server. Both servers are Windows 2008 R2 x64. The backup is full to disk ( 1 per week ) and after that duplicate to tape. The other days of the week is differential.

The speed of the full backup is 152 MB/min

The speed of the differential is 1200 MB/min

What could be the reason for the slow speed on the full backup. Is there a way to improve it. The servers are connected on 1 Gbps. The connections is OK. If I try to copy files between the servers the speed is OK.