Friction on the ground is directly proportional to surface gravity, so moving the same load over the same ground at the same speed on the same vehicle takes 10% less effort from your tractor. Especially with animal traction, endurance isn't linear, and I don't have the experience or training to say how that improves, but it's probably more that 10% - maybe as much as 20%, although that seems extreme. Max load, OTOH is linear: for the conditions specified you should be able to increase the load moved by 11%.
Faster is trickier. Not only would the animals have an optimum pace and cadence that isn't [I believe] all that dependent on the load, but friction is a function of speed, so you'd be back to the same effort when you were going only some 3-5% faster, for the same load. Getting 10% more force to the same point is probably of more benefit than getting there 5% faster.
BarryKirk wrote:How do you factor in the gravity level of 0.9?
I don't think we have enough real world experience to even guess that.