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...

There's no code which controls the aliens' speed; that doesn't mean it wasn't intentional.

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

But basically one alien moves every video frame.

It was intentionally decided that "it's not a bug, it's a feature!"

"We don't make mistakes, just happy little accidents."

- Bob Ross

"When you hit a blood vessel don't say 'Oops,' say 'There.'" —attending surgeon to resident

This is true: I was holding a retractor on the other side of the operating table.

I'm just a little sick of this getting trotted out as a factoid when it's patently ridiculous, like they never noticed it or that compensating for it in some way was impossible. There are actual unintentional mechanics like the Nagoya Shot, unexpected technical skills like the rainbow finish, or one of the earliest examples of RNG manipulation for UFO control, that are much more interesting to talk about.

You could share the cool things you are interested in without hating on the things other people think are interesting, though. I'm not familiar with any of the techniques you mentioned and would be curious to hear - sharing them would broaden and enrich the discussion. I can understand having a pet peeve, but that's a matter of your preferences - not something anyone else did wrong. Additionally I think it's founded in a misunderstanding - we all get it that the designer left it in on purpose, that's why we think it's cool.

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.

It was not intentionally designed. It was intentionally left in. The first statement is what is cool.