Throwing this out to see if there are any ideas. I recently had gigabit internet installed at the house. Testing, my 2011 Mac mini will only reach 136Mbps down but will get 850Mbps up. (over WiFi it will do 136Mbps down and 300Mbps up) Sorry for this next part... my Windows machine (work) will hit 950Mbps in each direction using the same exact cable. I remove the cable from my MAC and plug it into my WIN7 machine. My MacBook Pro, same issue only worse. It will top out at 90Mbps down and 600Mbps up. All of this is ethernet, CAT6, direct to the provided gateway. I only have two MAC's to test with, one on Sierra and one on High Sierra. The cable used is only 12" long, I wanted to give the machines every chance I could. I will go ahead and say it before anyone else, I know 136Mbps is plenty but if it could keep up with my WIN machine it would be better. Wonder if age is a factor. I should mention I use an SSD and have 16GB of RAM installed.