Do you have same low pressure issue on cold when using on-board pump?
Is toilet affected by this low pressure/flow issue also????
City water feeds main cold water line off of which sinks/showers/toilet & water heater 'T' for cold-side supply. (pump discharge ties into this main line)
Cold goes in bottom of water heater and then out top to feed main hot water line off of which sinks/showers 'T' for hot-side supply.
Cold water low pressure/flow issues should affect hot water pressure/flow also.
Might need to disconnect cold line to sink.....place in bucket and then turn water supply on. Check pressure/flow into bucket. Good flow-----then faucet could be plugging on cold-side port.
Try at each faucet/shower.
Has to be something got into main line and clogged up cold side faucets.
Plugged system filter would affect hot/cold
City water inlet problem would affect hot/cold