What does it take to get 1.2 Gb download speed over Ethernet?

I received the downstream upgrade from 1 gig to 1.2 gig a few months back.  My d/l speed to my Xfinity Gateway (CGM4331COM) is around 1,300-1,400 mb/s.  However to my devices using the 2.5Gb port or even the 1 Gb ports according to the device speedtest is around 940mb/s.  Is this because my Ethernet adapter is only 1 Gb?  If I had a 10Gb adapter would I see device speeds over Ethernet of 1.2 Gb?