What's the fastest connection speed / shortest elapsed time using Migration Assistant?
Gigabit ethernet to gigabit ethernet. Next would be FireWire 800 to FireWire 800. Thunderbolt target disk mode is testing to be equivalent to FireWire 800 in target disk mode. 

It looks like the OS can keep a gigabit link fuller than target disk mode is able, even when the target mode link has vastly higher bandwidth available.