I have a problem in my bathtub relating to water volume or pressure being extremely low. The rate of water from the faucet to fill tub is so low it takes a considerable amount of time to fill tub. Subsequently, the same issue is affecting the shower flow. I have a 1970's bath with Eljer dual water valves for hot and cold. I have replaced these more than once thinking this had to be the issue. No luck. All other sinks have much higher volume/pressure.
Can you speculate what is causing this issue?