Rockets can reach higher speed in outer space for the same acceleration. This is because of the lack of atmosphere and lower gravitational force.
There is no atmosphere in space. Thus there is no air resistance slowing down the rocket. In space, the rocket will gain speed as long as the engine in working.
The farthest the rocket is from Earth the weaker would be the force of Earth's gravity. When a rocket flies against force gravity, it can increase its speed faster in space.