That is a great game! It was very influential too.

No scoring system, just a rank. I still enjoy playing once in a while.

There is a slow down that happens when multiple enemies are destroyed and the computer cannot compute all the particles.

Someone fixed it a while back and the game runs at a perfect 60/50hz. I played that and the game seemed stale, not "alive" like the original.

Same with an arcade game by Williams: DEFENDER. The slowdowns are awesome and add to the game experience in ways not really foreseen by the original creator.