I don't actually use a power splitter. All of my splitting has been by hand. a neighbor rents one once a year. And the rental always starts and works well. And tows well.
I looked at a splitter at the local building supply store yesterday. I didn't see anything with a speed recommendation, but the online manual listed 45 MPH.
It had 4.80-8 tires 4P.R 62M Load Range B Max load 590 lbs at 60 PSI.
No labels on the tires listed a speed limit.
That is a common utility trailer tire that should be fine for highway speeds. As I mentioned earlier my small utility trailer has the next size up, 5.70-8 tires, which I upgraded to Load Range D, 1075 lbs each tire, and I've towed the trailer coast to coast. The biggest problem was that it was invisible when towing behind a rental truck.
Now, that splitter had an odd integrated hub in the in the rim. So, any tire maintenance means taking the whole wheel and hub off. Still, hopefully the rim/hub is OK.
One of the notes I saw on the internet:
If you tow it to any real distance at any real speed (over 20 MPH) it probably would not hurt to swap the wheels out for a 4x4 hub and use wider tires. You really should not tow any log splitter like this over 35 MPH and go SLOW around corners so you do not flip them.
It may be that it isn't the tires or the hubs that is the limitation. Rather the trailer may be tall, narrow, and top heavy. So emergency braking, or skidding around a sharp corner could cause unpredictable results. In fact, shorter tires may be a benefit.
At a weight around 600 lbs, most vehicles should be able to stop it without trailer brakes.