Most of the time the problem usually isn't the water heater at all. Shutoff valves to the hot side of faucets are often turned down to help prevent scalding.
Cut off the cold water (intake side) and unhook the connection. Take a flash light and look down the intake side for any "trash". Sediment equals low flow.
If you have pressure on your cold water, then the most likely problem is that it is rust from the inside of the tank on the water heater. If you have an old water heater then it starts to build up rust on the inside.
