Interesting. Could you please share the Samba config for the Time Machine shares? And over what medium do your Mac machines connect to the NAS? Wired or wireless?
Connections are a mix of wired and wireless. The NAS box has 10GbE to my UDMPro.
See my other comment for how the datasets are setup. I forgot to mention that I applied a quota to force each client to prune its history to avoid consuming all disk space.