Interesting Space Invaders Trivia:

The game speeding up as invaders are eliminated was an unintended consequence of the hardware running full speed to draw all 55 invaders. As invaders are eliminated the draw calls finish faster and the game speeds up. There is no code in the game to throttle the speed. The 2 Mhz 8080 is always drawing full speed. It's delightfully serendipitous this happens to ramp up the difficulty as you near the end of each level in such a compellingly perfect way. (https://www.tomshardware.com/video-games/retro-gaming/space-...)

I've watched some interviews with the game's programmer Tomohiro Nishikado and, although translated (so subject to garbling), he seems to confirm this was a 'happy accident'. He indicates he set the max number of invaders based on what the hardware could draw but there was no intent to have the speed ramp up. Of course, he noticed that it did this during play testing but decided to keep it that way. Arguably, it's one of the most compelling aspects of the game. Modern emulators have to add code game-specific code to limit the speed or the game plays too fast. Leaving no CPU cycles unused is the sign of an elegant design.