They are not slow, it's that many airports are very badly designed with regards to ground operations, without proper vehicle parking spots close to the main terminals and without proper vehicle paths like in reality. Sometimes you just see them, but they are not included in the AFCAD.
This result in longer than normal travel times of the vehicles, because in order to minimize the chance of conflicts with AI airplanes and behave realistically, the vehicles will give preference to parking spots of the *vehicle* type, and using paths of the *vehicle* type, even if they are longer. They'll only use taxiway if they have no other choice.
