WHen it comes to good showerheads, you tend to get what you pay for. Did you get that flow through the showerhead or the tub spout? All showerheads sold today are flow restricted to be less than 2.5gpm. If you got that flow through the tub spout, you have other problems. If it was the showerhead, it may be normal. A typical 1/2" valve may flow in the order of 5-8gpm, depending on the temperature you select, and, some will flow less. A valve designed for high flow would be at least 3/4", and maybe larger, depending on the desired flow. Many 3/4" valves are not much better than a 1/2" one, but the higherst flow would be about 17-18 gpm.
That may all be secondary if you don't have a large enough water heater!