When doing the first Time machine backup to a network drive we saw average speeds in the 2-3Mbps range.
However, when we shutdown the Mac then resumed the backups, the speeds dropped dramatically (to 30-80 Kbps)… enough so that the estimated time to completion skyrocketed from 15 hours to 16 days.
Presumably, Apple does certain first-time back optimizations for speed, since the first backup is invariably large.
So the lesson is: do not stop the first backup. If you must, it is probably faster to just delete the partial backup and restart from scratch.