What would make the water from the bathtup faucet run slower?

Question

Up until a few days ago it ran full force, now all of a sudden it barely comes out at all? How can I fix it?

Answer 1

replace the cylinder of the tap, or maybe you have limescale build up in the boiler, but because its just happened its most probably the first one, the tap washer could be causing this as well,

Answer 2

You apparently have an obstruction, which could be sand (a small rock) in the system.  It is only at the tub spout, so is localized, most likely at the faucet.  If nothing has become trapped in the faucet (funny, how did the shampoo cap get in there?), you will need to flush the system out.  

You do not define the type of valve you have, so you will need to get your owner's manual (go on-line to the manufacturer).  Shut off your water.  

Remove the stems or cartridge, have someone cover with a bucket (you didn't want water EVERYWHERE, did you?).

Turn on water and flush system.

Turn off water and replace stems or cartridge.  

Turn on water and test faucet.

Or, if you have too much money, call a plumber.