The quicker a food is frozen, the smaller the ice crystals that will form. Large ice crystals damage the cell walls of food so that when it is thawed, liquid is lost, including soluble nutrients. Meat especially loses so much moisture that it will be dry when cooked if frozen too slowly. This is why many freezers contain a fast-freeze compartment for the initial freezing process.
Thawing, however, should take place as slowly as possible since rapid thawing also leads to a loss of moisture and dry, tasteless food.