I've had the Terra Speed for over 2000 miles. Predominantly on gravel rail trails. I looked at them a couple weeks ago and see no reason not to keep rolling them.
I suspect the rolling surface and the power output play a huge role in tire wear.
600 miles on mixed hard surfaces (for me, broken chip seal and hard pack dirt) at 200-1000 watts kills the tread life. At 0 watts, they still look fine on the same surfaces. I suspect that running them on softer gravel surfaces would help a lot too.