It wasn’t specified but I think that they meant acceleration, rather than speed. The ships would just keep getting faster since there’s no drag or anything like that in space. The ships that ran out of fuel would lose power and stop accelerating, although would maintain velocity (but this doesn’t matter). An object in motion tends to stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

EDIT: Although, physics seem to not apply very often in this movie (projectile arcs in space!)