There are two "ranges" of fiber. One end causes the colon to slow down and the other causes it to speed up. Bran, corn and oatmeal cause it to speed up and beet pulp and tomato pomace (in dog food) cause it to slow down. You want the fast kind for constipation.
Fibre is basically anything insoluble (doesn't dissolve in water), so the more of it, the bulkier the stool, but if it creates a sort of intolerance reaction, it'll speed things up..