They close up, but mostly at the turns and after the HW tank. I've had galvanized in all my homes and never saw a leak. This house has 50 years old galvanized and the pressure is fine. I re-piped the scaled up 60 year-old galvanized in my last place, a
2-flat. Didn't bother replacing the verticals in the plumbing wall, as they looked good inside. Worst parts were the first el in the supply, and all the HW supply up to the veriticals. That was 30 years ago and I still have family living there with 90 year old galvanized in the walls and good pressure and no leaks. If I ever get a low flow problem in this house I know where to look. I've always been on Lake Michigan water, and I understand galvanized might have more problems elsewhere. The OP has a problem with one shower. He should find out why that shower is slow. A sink faucet screen with some pieces of scale in it can almost stop the flow, so the same can happen with a showerhead. Don't know if he has a tub shower and the tub faucets run strong. He didn't say. That's par for the course.--Vic