I just bought a house and the flow seems kind of slow in my master bath tub. House is about 12 yrs old. I'm not sure what kind of faucet it is; but basically turning the round knob controls the flow, and there is another knob on top of that one which will control the temperature. There does not appear to be any leaks or anything.
Sounds like a Delta. Does the temp control work correctly??? Make sure all shutoff valves are full open, cycle them off/on to be sure. If that does not help, the cartridge could have a partial clog or mineral buildup.
Since they are at a tub, there may not be shut off valves. You might try to shut off the water to the house and disassemble the faucet and see if it is grunged up or if there is a seal that is blocking the water flow.