Are you on a well or city water? Does the water softenenr have a bypass setup? Have you tried bypassing it?
If you are on a well, any clue what the water pressure is? If yo udon't have a pressure gauge, pic one up and see what the pressure you have is.
How old is the place? Could any of the pipes be galvanized steel? Those can rust from the inside out and cut the available volume to a trickle.
Have you taken the aereators off of the faucets to see if they are clogged up? How about taking the showerhead off and see what flow you get?