In fact, the slowest component dictate the maximum speed available... This is thrue for 95 % of computer system (hardware, software, Modem speed, internet speed,...)... It's the same from the road.. if you have 2 old grand-papa driving at 60 KM/Hrs on the road "side-by-side", you can have a BMW, or more, you will be "jam" behind them... because the slowest dictate the maximum speed !
in the NAS, it's 99 % of the time the network !