The slowest pitch in softball is the change-up. It is used to batter swing off-balanced. Off-balanced batters tend to hit more pop-fly's than balanced batters. Pop-flys are considered the easiest out in softball.
There are two different kinds of softball, fast pitch and slow pitch. Rather than try to explain I will point you to a wonderful link that will explain all. http://www.sportsknowhow.com/softball/dimension/softball-fast-pitch-field-dimensions.html
In terms of actual speed, a baseball pitch is much faster then a softball pitch. However, a softball pitcher throws from a shorter distance so it often looks just as fast if not faster then a baseball pitcher.