The Hero Casting Ratestat determines how fast the hero will heal itself, cast, upgrade and repair defenses. However, the Summoner's minion summon rate is not increased.
Wielding equipment that lowers your casting rate will not penalize it below zero.
In difficulties Easy, Medium and Hard, Hero Casting Rate is nearly instantaneous during Build Phase. In difficulties Insane and Nightmare, casting rate is the same in both Build Phase and Combat Phase.
Increasing this attribute decreases the amount of time it takes to build, repair, and upgrade defenses; it does NOT affect weapon fire rate.