Water pressure weirdness

[hermetic]

The water pressure has recently dropped significantly in the bathtub which has a round 1-handle faucet. It stays the same for hot, cold, and all points in between. The faucet is on an outside wall and about 2 feet away outside is a standing pipe connection for the garden hose. The pressure here has also dropped. But, the pressure is normal in the bathroom sink across the room from the tub. Likewise for the kitchen sink.

Went down to the basement to look for leaks or anything unusual but all seemed fine. The water here is heavily mineralized. Calcium buildup is common. I’m guessing the pipes are 40-50 yrs old.

I was wondering if you had any suggestions for things I could try before I do the obvious and call a plumber.
In gratitude…