How to measure local Ethernet speed

As a rough idea - your speed must be somewhere between 47 and 100 Mbps. Must probably it is 100 Mbps.

If you want to test, get a second computer and take the connection out of the router and plug it into that. Then you can start a transfer from one to the other and measure the speed.

One of the most common tools for making such a transfer and measuring its speed is called "iperf". You can install it via HomeBrew (if you have that), or you can download the Mac binary from their webpage.

After installation, start Terminal.app on both computers and run:

iperf -s

on one, and:

iperf -c 1.2.3.4

on the other. Replace 1.2.3.4 with the IP address of the first computer.

After a short while it will tell you the bandwidth in Mbits/sec.

Found an app called LAN SpeedTest, which unsurprisingly is well-suited to the task.

Running it on another computer, connected directly to the router by Ethernet, and writing/reading a chunk of data to a shared folder on the Mini, it gives Read speeds of c.60Mbps and Write speeds of c.50Mpbs.

Why it's so low is, of course, another question.