Anytime you shut the water off, the surge in the pipes will dislodge accumulated crud in the lines...this is totally normal. If you have galvanized steel water lines, or steel nipples in the supply, eventually, those will rust from the inside. As they rust, the ID of the pipe gets smaller and smaller (rust is bigger than elemental iron). It's more likely a volume issue than pressure since a soda straw and a fire hose can have the same pressure, but the volume available out their ends will vary radically. You may also have some accumulated crud in the showerhead or aerators. Some valves have inlet filters, and those could be clogged. On the shower, take the shower head off, turn the water on and see what kind of flow you get out the pipe. If it's good, then suspect the showerhead. You can also unscrew the aerator on any faucet that is slow, and see if it improves the flow...check and clean it if necessary. If it is a result of galvanized supply pipes, you may be in line for a house repipe.