I take it to mean the horse will stop being as smooth once you get to a certain speed.
How un-smooth will vary quite a bit from horse to horse. One of my TWH does a pace that is not as smooth as her running walk but is still very easy to ride. My other TWH does a god-awful hard pace that is almost impossible to ride.
The only way to know is to test-ride the horse at different speeds. Make sure you set the speed, don't let the horse just follow another horse who's setting the speed. That's a mistake I made once when I bought one of my Walkers.