There are two pipes on the top of most conventional water heaters. One is for cold water in, the other is for hot water out. Plumbers are SUPPOSED to put a valve on the cold water supply to the water heater, but some are too cheap. If that valve is not open all the way you might get low hot water pressure. If the heater is old it could have a lot of sediment in the bottom and THAT could reduce hot water flow and slow the production of hot water. Some homes are equipped with a water pressure regulator where the water comes into the house. If that is turned too low it could affect the flow of water everywhere. If it's none of these, you probably just have small pipes or the pipes are full of hard water deposits, limiting water flow.