Usually it's caused by a big box store in line filter plugging up. If you don't have one, you may have a water softener/iron filter etc that is getting plugged. Another thing is piping being too small to carry enough water to supply all the fixtures being used.
Is this something that just started happening or has it been going on for some time?
We've always had this problem. The house was built not too long ago(1975), but we've found it wasn't up to the best standards, thus entirely possible that the lines are smaller than they should be. That would not be an easy fix.
You might be able to crank the pressure up some on the pump. That always helps some. If the pipes are galvanized which they could be from the 70's, they could be coated with iron, sulphur and hardness. That always takes away the flow too.