8Dweb
Networking and data flow are very simple concepts.
for example
ISP switches and routers serve multiple clients; each client has a 1GB port speed with which the server is presented. A switch with 48 port speed should probably switch a lot, but the uplink is usually 10 x port speed, (1GB x 48 ports and 10GB uplinks) so for example, 48 GB speed on client's ports and 10 GB uplink port.
Most clients will not run red hot; the port, instead, will run them at 0.1 - 5%, so when a client decides to test its limit, most of the time, it will be able to achieve 1GB speed and would say... that it has 1GB speed.
In reality, when backups happen, especially to a different Data Center, and 20 clients decide to run full-speed backups to other DCs or maybe different operations that require data flow, each client would get a max of 500mbps for the time being.
You can consider another scenario where data on the network is limited, like data shaping 24x7, where a port is presented if 1GB, but it is limited to 250 Mbps transfer between particular hours. This is why you should test your uplink before considering you have x amount of GB available.
The above does not apply for the same rack or the same DC or between devices/racks with dedicated fibre (speeds for local are reliable and full speed most of the time). Most of the limiting happens between regions or different providers.
