How to find negotiated link speed?

[FreeBSD]

I have my Ethernet set for Automatic link negotiation. How can I see what was negotiated? How can I check the current link speed?
I ran a test:
time cp {3.5GB file} /dev/null
and it appears it transferred at a rate of 72.5 MB/s.
I believe 1GB ethernet can reach 125 MB/s.
Perhaps the NAS the file is on can't saturate the Gigabit ethernet?
I wish Mint would display a dialog stating the transfer rate when a operation is completed, not just during the operation, which changes anyway over the duration of the operation, and disappears when the operation is complete.
By my calculation, I'm getting a little better than 50% network speed.

[DJ55]

Open ethtool in Terminal and use it to check the speed on the NIC, as in ethtool eth0, where eth0 is the kernel name for the NIC. You should be able to get the NIC name from System Info. ethtool -h will also show you all of its various options.

You can also try opening LM's Network app and click on the little gear icon shown for the network you're connected to. It will show details of your connection and includes a stated link speed. That might be your negotiated max rate, I'm not sure.

[djph]

I have my Ethernet set for Automatic link negotiation. How can I see what was negotiated? How can I check the current link speed?

IIRC, ip link will show this. Granted, it's just going to show whether you've negotiated 10, 100, or 1000 mbit/sec

and it appears it transferred at a rate of 72.5 MB/s.
I believe 1GB ethernet can reach 125 MB/s.

It can't. In reality, you've got 10-15% overhead imposed by the ethernet protocol itself -- for the sake of discussion, assume that a sustained 110 MB/sec is about the best you can get (and that's probably in lab conditions)

[

Perhaps the NAS the file is on can't saturate the Gigabit ethernet?
I wish Mint would display a dialog stating the transfer rate when a operation is completed, not just during the operation, which changes anyway over the duration of the operation, and disappears when the operation is complete.
By my calculation, I'm getting a little better than 50% network speed.

You're getting ~70% (give or take) of the absolute ideal.
Your NAS is certainly a bottleneck (assuming it's one of those consumer models), as is any consumer-grade network kit (netgear,dlink,etc), available RAM in your machine, etc.