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 various technologies for transmitting Ethernet frames at a rate of a gigabit per second (1,000,000,000 bits per second), as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks, as a result of being considerably faster. The cables and equipment are very similar to previous standards and have been very common and economical …
Gigabit Ethernet - Wikipedia, the free encyclopedia