The tub in my master bath which is upstairs, has very poor water pressure. It was working fine, and then one day when I turned it on there was hardly any pressure. It takes forever to fill the tub. The shower and sinks work fine.
Grubby..You didn’t say what kind of valve you have.A single handle valve is more prone to a blockage than a 2 or 3 handle valve.The spout is the easiest thing to check so I would count that out first.Then I would start with removing the stems and fluushing the valve out.If the diverter for the shower is on the valve and not the spout then I would remove that as well.