There's no code which controls the aliens' speed; that doesn't mean it wasn't intentional.
But basically one alien moves every video frame.
- 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.