Water from bathroom basin taps flow changes
Since the hot water's flow rate depends on keeping the boiler pressurized, and this can only happen by feeding cold water to the boiler as you draw hot water, the total flow rate is limited by the flow rate of the cold water feed to the house.  You will see a similar reduction in flow rate if you open both the bathtub and the bathroom sink's hot (or cold) faucets, or any combination of separate water valves in the house.