Try to upgrade to vSphere 7, it should help. There are also 3rd party tools to do this as vRanger, esXpress, PlateSpin, NAKIVO Backup & Replication. With NAKIVO, you can use VMware’s CBT for your VMware VM backups. Changed data blocks are quickly identified and only those "increments" are copied to the Backup Repository. This results in faster and smaller backups.