Home runs can be as slow as 90 mph off the bat.   This suprised me, at first, as that is the speed of a pitched fastball, and we do not imagine that someone can throw a baseball over the outfield fence from home plate.  However, a good strong arm can reach home plate on a throw from mid center field, and the nearest fence portions, of small ballparks such as Fenway, are as close as 325 ft.

There were two home runs at under 89.5 mph off the bat, but both were in-the-park home runs. 😉 

The fastest home runs were slightly over 122 mph, off the bat.

HitTracker :: Home run tracking and distance measurement