A fast or slow approach to change

What are the factors to consider in deciding whether a fast or slow approach to change is best? What are the factors to consider in deciding whether a top-down or participatory approach to change is best? Describe a situation where you were either a change recipient or a change leader and a poor choice was made for at least one of these two decisions (use an example from an organization in which you currently work or formerly worked, or use a volunteer organization, a church, a sports team, a fraternity/sorority, etc).