With some exceptions (The Oklahoma City F5, the Jarrell F5) most really destructive tornadoes are moving quite fast. They end up covering more area with a longer track.
The deadliest Tornado in history, the Tri-State Tornado, was one of the fastest moving tornadoes ever recorded - 70 mph+
Maybe along the coastal areas, quicker storms mean less destruction....and the more inland, the greater destruction.
But I was thinking that a slower storm also allows a tornado to build to a large funnel and, therefore, create greater damage.
(But seeing as how I have never once lived in an area where there are frequent tornadoes (much less seen one), I profess my utter ignorance as to anything about them, except for what I read about them from experts or anecdotal commentary from others.)
When he first told me this story, I thought he was pulling my leg, but he has a clipping from a newspaper that talks about the return of his pants.