Somehow to what i've experienced on my side, i feel that the link speed that we share between PCs are like "see saw" / multi "see saw". Whenever one is utilising more, the other side will have trouble connecting to the net at its usual speed. Correct me if im wrong.
Because there is this thing called a Network Switch.
There are tree types of layer two devices, Hubs, Switch's and Bridge's.
Hubs, also known as multi-port repeaters, take the traffic from one port and repeat it to all ports. With a Hub every connected device must share the same bandwidth for that network segment.
Switch's, also known as layer two routers (although rarely does anyone use that description anymore), take the packet from one port, inspects it's header for the destination MAC address and sends it only to the port that contains that MAC address. If the switch doesn't recognize the MAC it puts it down all ports similar to the Hub. Since any host connected to a switch will tell the switch it's MAC, the switch will always know the MAC of all directly connected devices. Switch's can also communicate with other Switch's using Spanning Tree Protocol (STP) to share their MAC database's to prevent unnecessary broadcasts. In this way only the bandwidth of two devices are used for any particular connection.
Bridge's take traffic from one port and convert that traffic to a different protocol / format and put it down a different port. Going from Ethernet to Fiber Optic (Media Converters) is an example of a Network Bridge device. Bridge devices from one network segment to another were very common in the 10base-2, 10-base-5, and 10-base-T days.
To better describe bandwidth you have to look at Ethernet's structure. Ethernet is a bus protocol, packets go from one device to another all the way down the physical wire until it hits a terminator. Modern times we physically use a star layout with a central device connecting all other devices, but Ethernet requires that all devices listen and hear the same packets. Hubs do exactly this, so that while you may have four PC's and one router (five wires, one switch) connected at 1Gbps each, their being treated physically as one wire with a 1Gbps bandwidth limit. Switch's simulate this but do so smartly by dynamically connecting devices to other devices only when their talking to each other. Thus you have five wires of 1Gbps each, your total network bandwidth would be 5Gbps. Computer A could copy a file to computer B at 1Gbps, Computer C could do another transfer to Computer D at 1Gbps simultaneously. This is because all four of those computers have different physical wires, and its the wire that is rated at a bandwidth limit (1Gbps). Of course switch's have a limit to their ability, this is known as switching capacity and is a big factor with enterprise networks.