I still think the most interesting thing is that the game doesn’t intentionally speed up aliens, the game just runs faster when there’s fewer aliens.
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...
- Bob Ross
This is true: I was holding a retractor on the other side of the operating table.
There's an important game design lesson in one of the defining features of this game being an artefact of the hardware that the game designer accidentally bumped into. That's fascinating. And happy accidents like that are still an important part of game design today.
But basically one alien moves every video frame.