As @Royalty correctly notes, any one gig edge port can fully consume the gig uplink port.
So, what happens if two gig ports want to use the gig uplink port, as they can send 2 gig to the 1 gig port?
Well, that's very much an it depends answer. Assuming two edge ports each want to use their full gig, with "classical" TCP, the uplink port would queue packets, queue would fill, overflow would be dropped, TCP would detect the drops and effectively drop the transmission rate. In theory, each edge port would average about 500 Mbps, but TCP would continue to try to use a gig for each, so drops would be on-going. Actual usage, short term, would be very variable, and long term, and depends on multiple factors.
To avoid the queuing, on the uplink interface, you could run the edge interfaces at 100 Mbps or 10 Mbps, but you've only relocated the underlying issue. What if two edge ports want to transmit to the same 3rd edge port? What if the uplink port, still set at gig, wants to send to an edge port running at 10 or 100 Mbps?
What if, the edge ports are still running at gig, but you have multiple apps, on the workstation that want to transmit? I.e. how fast can the workstation send various app data to the workstation NIC? (Hint, probably faster than gig.)
In general, fixing network performance issues, by just adding bandwidth, doesn't always succeed. When it does succeed, it's often not for the reasons assumed.
Personally, one of the things that amazes me about networks, for half a century, there's been much development how to "share" a CPU or disk drives beyond FIFO, but now a quarter way into the 21st century, how do we share network bandwidth? Still, often the default is FIFO!
