The answer is inside the machine.

Space Invaders, 1978. Tomohiro Nishikado made it almost all by himself, down to the board it runs on, and that board was too slow to draw the whole army at once. So, in his words to Wired, it was designed to draw one invader every 60th of a second. At the start, it takes about a second for all of them to take a step.

Shoot some down and each invader's turn comes round sooner, so the whole army marches faster. The last one gets a turn every 60th of a second. The invaders never sped up: the game just had fewer of them to draw. Nishikado's verdict: "This makes the game more interesting and effective, and compensated for the board's lack of capacity."

The invaders, shots and timing in this film are our own simplified version of his rule, built in code; the one-at-a-time march is shown slowed down 10 times. Not affiliated with or endorsed by Taito or Square Enix.

Which arcade trick should we open up next?

Sources: Tomohiro Nishikado to Craig Grannell, "The 'Space Invaders' Creator Reveals the Game's Origin Story", Wired, 12 April 2023.
Sounds: our own, made in code.