How to fix Thunderbolt to Gigabit Ethernet slower than Wi-Fi Macbook Pro?
I think there is something going on with El Capitan and a most recent update...   My ethernet speeds (same network as an old Mac Mini) are 50x more (very seriously) than my iMac 5k.  I get about 70Mb/s on the old Mac Mini and only about 1.5-2Mb/s on the iMac.  I checked the network traffic, I checked the processing in the background ...  nothing.  That's changed recently (with nothing else changing) and I think it was due to an update.  Frustrating though.