All pipes in a house are connected, so when two outlets are turned on the water has to change from going in one direction to being divided and go two directions. However, the loss in water pressure should only be temporary . . . usually the system will re-adjust
If someone were to flush a toilet, the water pressure could drop accordingly. If you lost all water pressure, chances are that your main water line coming into the home has a huge leak or not enough pressure. The hot water tank is pressurized by the cold water pressure coming into the house. If there is no water flowing from the water meter into the home, you won't have any hot water either.
Unscrew the aerator from the end of the faucet where the water comes out and clean the screen. Sediment build up slows the flow.
The tub spout diverter washer has had the biscuit, replace it.
Bathroom sink use 5 liters of water.
Snake your drains
Sink is not vented properly and the water from the p-trap under the sink COULD be sucked out by the hydrostatic pressure from the toilet flushing.
Additional shut off valve, below sink or in basement. Incomplete pipes not tied into bathroom sink line.
The simplest answer would be that the screen in the spout is clogged.
A poor plumbing job. The pipes to the shower & sink share a pipe to the water heater and the pipe is not large enough. This causes the water pressure in the pipe to drop when the hot water is turned on at the sink while the shower is also on.
Unscrew the aerator from the end of the faucet where the water comes out and clean the screen. Sediment build up slows the flow.
Replace the faucet.
remove aerator
The problem could be as simple as a clogged screen in your faucet. It also may be a low flow gasket which is in the faucet to reduce water usage. Has this alwys been a problem or is it a new problem?
The under sing shutoff valves and or the fill lines to the sink are clogged.Sometimes you can flush the lines and everything will work fine.Other times you need to replace the lines.Be sure to remove the aireator and check the flow first it might be the whole problem.
In order for water to reach your sink, pressure needs to be exerted for it to beat gravity's force.