your Thiebault thesis was the subject of some discussion here recently. I have downloaded it but not had the chance to read it in detail yet.
Regarding ox carts, yes they only move as fast as oxen are wont to do, which is rather plodding. Ox carts were broadly used in other places, so it would be interesting to correlate findings. For example, I spent much of my childhood in Winnipeg and ox carts were part of the local history. This link http://www.saskschools.ca/~gregory/cart.html indicates that the Red River Cart travelled about about 20 km or 12 miles per day (note that there is a typo in the website, reversing the metric and imperial measures). If you reckon that the cart could be driven 10 hours a day, then you get a rate of 2 km/h. The terrain was decidedly non-mountainous. The region is the bottom of a massive glacial lake from the last ice age and so quite flat with very fine particulate mud. The flatness made it suitable for carts instead of pack animals and the mud made oxen more reliable than horses as draught animals.
I think that 3 km/h from oxen might be reasonable when not hauling a load, but seems a bit aggressive if in harness.
Hope this is some use,