Why is rapid cooling needed to form glass?

i'm asking about glass that needs to be cooled rapidly, like tempered glass.

Rapid cooling is needed to form glass, specifically tempered glass, to make the glass stronger and safer.

Unlike regular glass which breaks when undergoing rapid cooling, tempered glass is ductile. It is not brittle and its ductility is a result of undergoing the rapid cooling process. 

Through rapid cooling, tempered glass develops a core that has a large amount of tensile stress and a surface that has compressive stress. The compressive stress on the surface of the tempered glass makes it stronger than regular glass. It can also be broken but when it does, it breaks into small granular chunks that are less likely to cause injury.