I have a LAN that has 4 Macs and a router / switch to connect them all.
My Ethernet LAN speed on my iMac is pathetically slow. I additionally have a MacMini, MacBook and G4 in my LAN, they all go way faster. For measuring I used a 220GB file to copy from one machine to another. While between the MacMini and MacBook this estimates to take about an hour, when copying to my iMac this measures at about 15 hours.
I then unplugged the cable from the iMac and plugged it into the MacBook to check if it might be a cable, and the speed was fine. So the cables seem to be fine.
I then connected the MacBook directly to my iMac bypassing the router and the speed was fine too, so the iMac port seems to be fine.
So, I’m thinking it’s something in the settings. On the iMac, I have two partitions, one running 10.11.6 and one running 10.6.8. On both systems I got the same results. When looking at network Utility, it shows Gigabit.
I reset the router and switch, restarted the iMac and am out of ideas right now.