My brand new Lenovo P40 39" 5K monitor has 1GBIT ethernet in the USB-hub.
Max throughoutput I get is 300mbit per second over this with a thunderbolt/usb-c cable in between, I find this laughable.
I managed to get 1Gbps (minus overhead) with the Dell version of the display. But it is highly variable based on what else my CPU is doing, which is my experience broadly with USB ethernet.
Anectotally, the 49" ultra wide curve displays from Philips with ethernet in the base at work provide real gbit throughoutput.