I have seen hot water pipes clog up where it screws into hot water heater check pressure at hot water drain valve on heater and see if it is alot higher pressure than another faucet in the house.if it is check for clog in pipe.
The aerator at the end of the spout needs to be unscrewed and the screen and/or water saver cleaned out. Before rescrewing the aerator back on, run the faucet and see if you have full pressure. If not, the spout itself sometimes gets plugged too.
Bad washer in the faucet partially blocking the flow. If it is both hot and cold, it is probably just sediment in the screen in the end of the spout. The tip unscrews and can be cleaned. If the faucet is old enough that it doesn't have a screen, it may be mineral buildup in the supply lines or at the shut off in the line.
If the pipes are old, it's most likely that corrosion has built up inside them. Some of this buildup may have become dislodged and clogged the path to the low-pressure faucet.
Probably someone or something else using water that drops the pressure. The shower head will be a lot more sensitive to pressure drops than other faucets in the house.
Could be a partial blockage in the supply lines to the faucet. Could also be a different size of supply line. The faucet valves may also need cleaning.
A leak in the supply pipe.
Air in the water lines to the upstairs. Not enough water pressure.
check for leaks
Lots of stuf can cause it including sediment or a blocked supply line
I would purchase a Watt water pressure guage from your local home improvement or hardware store and check your water pressure at your faucet. If water pressure is greater than 40-45psi your regulator needs to be turned down where the water comes into the house.
Possibly a pressure reducing valve installed after piping for first faucet, which would cause situation describing.
Stopped at all faucets? Didn't pay the water bill. If in cold climate, the line from the outside to probably the water heater has probably frozen.
With no air in the system anywhere, there would be nothing to hold pressure, so the slightest leak would cause the system to go to zero.
Do all the faucets do this or just one. if its just one faucet its a problem with this fauset. If its all the faucets in the home its a problem with the main water line coming into your home.
Most of the newer kitchen faucets have "flow restrictors" to limit water use.
Some causes for a sudden water pressure drop: 1) A big leak on the main supply line. 2) one or more faucets or shower heads opened on the same water line. 3) A utility water pump failure.
the pressure for 5 gal per minute would be affected by pipe size.1/2" water line at 5 per min would be about 60 psi