For school, I'm doing a project where we choose a topic about anything, ask questions, and learn about it. I chose my topic as fuel efficiency of airplanes, and one of the things I want to research as part of that topic is fuel efficiency of planes on the ground and see if we can cut down fuel burn at airports. While I was exploring that sub-topic, I thought about tugs and if they are capable of going faster than 30 kts. If they are, we can use them more than just for pushback and potentially cut down on aircraft fuel use.
This AC covers a lot of the regulations around towing aircraft and according to the regulations
Towing Speed. Towing speed should not exceed that of walking team members. Towing speed should not exceed the safe operating speed for the conditions or the published speeds established by the TLTV operators published procedures.
In other words you can do either as fast as your operator says you can or as fast as the tug manufacturer says you "safely" can. The AC goes on to discuss a lot of other things but also mentions this tug in their section on tow-bar-less tugs. According to their website:
Unique hydrostatic drive for moving aircraft on the ground at up to 32 km/h [~20 MPH] – without compromising safety
This is about the same speed that Terry discusses is the maximum taxi speed in this answer so you don't really gain any speed. Depending on the aircraft and airport in question you don't want to cut down taxi time to much as that may be an important time during which the engines come up to temp.