Fast local backup - Very slow backup via Network
You could attempt to backup up the database using compression instead. As long as you have the CPU avaliable, It will definately reduce the time (around 15%-20%) that it takes for your backups to complete. These backups also take less time to restore as well.

Good Luck,

Hope this helps.

Good resource: http://blogs.lessthandot.com/index.php/datamgmt/dbadmin/testing-backup-compression-in-sql-server-2008/