Answer Description

Although the user's internet access is capable of up to 1 Gbps (Gigabit per second), their home network still only supports Fast Ethernet (100 Mbps). 802.11g is only capable of 54 Mbps and switching to the 5 GHz doesn't provide enough information to determine the network speed. The only realistic scenario is to check the router, NIC and ethernet cabling to ensure all are capable of supporting Gigabit Ethernet speeds (or higher).

Wikipedia

In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use in 1999, and has replaced Fast Ethernet in wired local networks due to its considerable speed improvement over Fast Ethernet, as well as its use of cables and equipment that are widely available, economical, and similar to previous standards.