Why Low Water Pressure in Just One Fixture?

I live in a 2-storey house that is 25 years old. Most of the plumbing fixtures are also 25 years old.

A few days ago the water pressure in the main upstairs tub and shower became very weak. It was still fine elsewhere, including the two sinks in the same bathroom.

I removed the tub spout to investigate, and the water pressure coming from the pipe was weak. What is the problem and how do I fix it?

Yesterday the pressure in a sink in a different bathroom just about disappeared. Is this problem related to the previous one?

What type of piping does it have?
Copper or galvanized?
anyting changed recently?

Take the diverter out and see if you see a possible problem.