QoS can help. You need a managed switch though.

But like Matty said, in a home environment, the bottleneck re internet speed is typically at the modem, i.e. the speed provided by the isp.

The other place that a bottleneck could arise is at the router / switches / devices. Some older devices have ethernet rated at 100mb/s. Generally, and given the wide availability of 1gb internet, you want all devices to be capable of at least 1gb.