Film speed is a rating that measures how quickly the film will respond to light. Faster films have higher values in what are called ASA ratings. The problem is that the higher speed films may not always give the best results. The general rule of thumb is that the higher the speed, the larger the grains in the film.
Grains are the photo-reactive crystals in the film itself.
When they reach a size where you can see them in a print, they are too big. You can see grain when you blow up a standard picture to a wall sized poster. And when enlarging just a section of a photograph as typically done in astro-photography. Higher speed films are also more susceptible to light pollution or the background lighting. But if you are in a relatively dark area, the sky glow will not be too much of a problem.