BobVila.com

I have a feeling it is not so much a loss in pressure but more of a volume problem as the sinks have a 1/4" opening (seat)and the tub has 1/2" -3/4" opening and here lies the problem.

Many installers use 5/8 OD (1/2" ID) copper tubing and connect a tub spout with a larger opening then the supply pipe can handle so water basically dribbles out.