The frame rate stays the same (e.g. the player always moves the same speed). It's more that only one invader gets moved on each frame. So the fewer invaders the

The frame rate stays the same (e.g. the player always moves the same speed). It's more that only one invader gets moved on each frame. So the fewer invaders there are, the more they get moved. It's deliberate. I guess they weren't doing a whole-screen-refresh (with double buffer) in those days, just turning pixels on and off to move the objects. So then there's a limit to how many objects can move per frame. E.g. the player moves by blanking our pixels on one side and adding them on the other...