A study has shown that a baseball will lose about 1mph of velocity for every 7ft of distance traveled. When asked about a softball since it is bigger and will be more affected by friction the author replied with since the softball is heavier it also loses about 1mph for 7ft of travel. This may be where the add 5mph to a pitch comes from with the slow radars .
The reduction of speed after release is significantly affected by spin rate. Losing 1 mph for every 7ft in softball indicates a very weak spin rate. A pitcher with great spin (> 30rps) will lose 10mph or more from release to final velocity, which is the reason for the "late break" often observed in elite pitchers.