A good rule of thumb is the following in terms of performance in descending order - but your mileage based on the actually underlying hardware that backs each storage solutions (SATA vs SAS, rpm, # of spindles, controller type, software, etc).
1. vmfs on iSCSI or FC
2. vmfs on nfs
3. cifs/smb
Other items that help overall backup performance is ensuring the VMs being protected are upgraded to HW7 and the snapshots can be hot- added to the appliance (the VDR logs will tell you if this is the mechanism it is using or it is backing up over the network- which would be slower)
