iMac ethernet not functioning as expected

My iMac 27" 5K (2017) is experiencing problems with it's ethernet connection. I have a broadband connection which has theoretical speed limits of 950 Mbps download and 500 Mbps upload (the fastest I have ever experienced on it is 550 Mbps, both ways, funnily enough). When I connect my iMac via ethernet, I consistently get speeds of 95 Mbps (both download and upload). However, when I connect the iMac via Wi-Fi, I get connection speeds of 350-400 Mbps (both ways).

I thought there might be a problem with the cable I was using, so I plugged it in to a laptop to see if this was the case. After doing a speed test, the laptop consistently reached speeds in excess of 500 Mbps (both ways). This leads me to believe there is a problem with my iMac's ethernet hardware.

Is it possible that some software quirk has capped my ethernet speed (I have tried restarting my iMac and this did not fix the problem). Could it be craftsmanship problem with the port? I am keen to hear possible causes of this strange problem.

Yours sincerely,

Henry