Why is my water flow slow in the bathtub and shower but ok every other sink and faucet?

My Mother-in-law owns a house about 30 years old. It has good water pressure on the outside faucets, and all inside sinks. The bathtub and shower faucet is so slow. If you have the shower on it falls straight down and does not shoot out toward you. The bottom bathtub faucet is no better. It barley comes out. Please give me some ideas.