The Commodore 64 is a beautiful machine, but there are some things it wasn't designed to do. One such thing is animate a lot of different moving objects at once, all of them independent of each other, frequently overlapping, sometimes colliding, always changing. In other words, the Commodore 64 just doesn't have the hardware for a game like Galaxian.
Of course, the Comodore 64 seemingly doesn't have the hardware for a lot of games that were published, and some of them are very well done despite the system's limitations. Those games were in the hands of programmers who either knew tricks to make the hardware do more than it was designed to do, or had the time and patience to artfully optimize and compromise. C-64 Galaxian's programmers apparently didn't have any of those luxuries. While the graphics do represent the arcade game well enough, gameplay is lackadaisical and animation is jerky. Even the sound, which any Commodore game should excel at, is minimal at best, and rather annoying at worst.
Sad to say, just about every other home version of Galaxian is better, even the Apple II and Atari 2600 versions. Save Galaxian for one of those systems, and use the Commodore for games it actually knows how to play.