The speed-up was an accident
Fewer aliens on screen meant the hardware could redraw them faster, so the fleet sped up on its own as you shot it. Nobody designed that. Players liked it, it stayed, and now every rebuild copies a limitation from 1978 on purpose.
You feel it most with the last two. They move at roughly triple the pace of a full formation and they are the reason plenty of waves end one alien short.
The bunkers erode
Four of them, and they are not shields with a health bar. They are made of blocks, and every hit takes a chunk out of wherever it landed. Your own shots do it too, so firing up through your own cover eventually leaves you a hole to be shot through.
Playing it now
Arrows to move, space to fire, or the three buttons underneath if you are on a touchscreen. Only a few shots travel at once, so it rewards aiming over spraying. Clear the outside columns early and give yourself somewhere to dodge to.
The saucer that crosses the top is worth a lot and appears rarely, which is exactly the kind of decision that gets people killed.
If you want the other arcade rebuild, the wall at google block breaker is a slower kind of pressure, and the maze at google pac-man is the one that never got taken down.
