Why isn't speed kicking in?

Speed-related traits kick in when the horse passes a Soul Strength skill check, and turn off if they fail it. The opposite is true for negative traits; it can pass a Soul Strength check to not be affected by something like “malformed hindlegs”.

Hell horses have much higher Soul Strength than normal horses, so they pass these checks much more consistently. Unicorns, bisons, and other animals that aren't horses, don't do these checks; the traits are always in effect for them.

This is why speed can seem inconsistent or delayed. It can take some distance before the skill check passes and speed kicks in. Be patient.