There is some code that controls the speed when there is a single alien left it moves faster right to left see the code at 00C2 in http://computerarcheology.com

There is some code that controls the speed when there is a single alien left it moves faster right to left see the code at 00C2 in http://computerarcheology.com/Arcade/SpaceInvaders/Code.html http://computerarcheology.com/Arcade/SpaceInvaders/Code.html But basically one alien moves every video frame.