It is the Router's Network Switch (i.e. its four LAN ports) which is either 100Mbps or 1000Mbps (1Gbps). With a 20Mbps internet connection, the higher port speed won't make any difference to your internet performance.
Where it will make a difference is if you transfer large amounts of data, or large files, between computers in your local network. However, all computers/devices connected by ethernet cable in the network also need to have network adapters that support Gigabit.