I have not noticed this with the half tracks, although I usually stay out of them.
At the core, the Kubelwagon and Jeep are extremely slow at accelerating. They should be some of the most nimble and quick of all of the vehicles.
I know the Bren carrier doesn't accelerate immediately, but it seems to accelerate more quickly than the Jeep and Kubelwagon. The Bren carrier has a bit of a delay but once it starts moving it really goes.