Water pressure on the supply side has nothing to do with the drain side.
If the pressure is suddenly low it could be a number I f things.
Is the pressure low everywhere? Shower, sink, outside bibs?
Buy a $20 pressure gauge and test your pressure at an outside bibs. Ask the city what the expected pressure is and see if it’s outside the range.
You may have a hidden leak that is causing a pressure drop too. Make sure no one is using any water in the house then look at the water meter. If it still spins you have a leak.