Windows Server backup of on Hyper-v host 2012 server taking a long tim

I am using Windows server backup to backup the VMs on hyper v host.  I am having an issue where it's taking a long time to finish the backups as it wants to do a full backup of each VM . Also , when the backups are taking place the VMs run very slow, For example, it would be so slow to access shares from a fileserver on one of the VM or a domain controller VM will not respond.  Also, do I need to turn on / off shadow copies on the drives that the VM vhds reside ?   I am starting to get the idea that only third party backup software will allow you to do incremental backups of the VMs which should take less time.