Your pipes probably have a lot of corrosion built up inside them. It's most likely that a piece of this corrosion became dislodged and is clogging the line going to the affected faucet somewhere.
The faucet is clogged
well if you have a plugged aerator or your water line is corroded then that is usually the reason a faucet has low pressure if its just one faucet.
Static pressure can be caalculated by simply measuring the height of the faucet from the highest fixture above this faucet and times the height by .434 for static pressure
no
No. It takes much more than water pressure to turn on the water faucet. At most, there is a leak if anything at all.
Reference pressure less the height.
A Roman tub faucet does have good water pressure because it is made to do that. How good this is depends on how other brands perform in this area.
you get a pressure gauge that screw onto laundry or garden faucet
Someone is stealing your water.
It is pushed by the water pressure in the system.
install larger pipes
Water pressure at any point in a household system is the same. ie, if it's 50psi after your PRV, then it will be 50 psi at any faucet (unless that faucet is plugged up)