backup speed

Well, as you mentioned, disabling AV allows you to achieve good speed. This kinda rules out an issue with the BE software. Have a look @ these posts as well:-

http://community.sophos.com/t5/Sophos-EndUser-Protection/Disable-on-access-scan-during-network-backup/td-p/2789

http://community.sophos.com/t5/Sophos-EndUser-Protection/Sophos-Slows-Down-Backup/td-p/40105

http://community.spiceworks.com/topic/84508-sophos-endpoint-security-and-backup-exec

http://community.spiceworks.com/topic/84594-backup-exec-and-sophos-endpoint-security

By the way,

that is my first BE2012 Server. I see that the default settings for a backup is to use the changedate and not the archivebit. I have changed it back to archivebit now. Why is that not a default value anymore?

Thanks guys