Turns out “Space Invaders” aliens sped up only because of a bug.

This really is one of those situations where “it’s not a bug, it’s a feature” holds true.

Space_Invaders_Second_Row

The original Space Invaders was written to have the aliens maintain a constant pace throughout game play.  But the machines took time to render the aliens.  As they were killed and removed from the screen, less rendering was required, which cause the play loop to complete faster and the aliens to speed up.

Space_Invader_Cabinet

You are probably wondering why the entire game didn’t speed up then, and in fact, it did.  In future version, this portion was fixed, but the increasing speed of the aliens was left in.