The base attack speed for heroes is normalized - Meaning no particular hero attacks faster than another. However items will augment the speed at which attacks/actions are performed. This is incredibly useful on heroes such as the Cleric, where increased attack speed will mean faster healing.
As of HvM Release 1.2, there is a "hard cap" of +65% attack speed. Meaning that anything above 65% attack speed will be the same as 65% attack speed.