Zombies can run fast and slow, it's just depending on how they are built. Animal zombies are the same way but more aggressive and harder to kill.
Normal zombies are very easy to outrun, but can commonly be encountered in groups.
Animal Zombies are as the name implies, Animals that are zombies such as dogs, lions, etc. Animal zombies are very ferocious and easily beat normal zombies in terms of how dangerous they are due to most animals having very sharp teeth and high speed.
Infected are Zombies that have recently been bitten. Fast and Agile, they run fast so the best way to kill them is to shoot them or slice their head off when they run up-close.