ive notcied the same. i logged into the xfi gateway and although it states the ports are Gb it shows the devices are connected at 100Mbps. this will bottleneck as 100Mbps is well... slow and not cabable of outputing the speeds needed.
If you see 100Mb connections it means the device at the other end or an intermediate switch has ports that are only capable of 100Mb. But, that is sufficient for any single device doing nearly anything intended for internet delivery, including streaming 4k hdr content. If you need to do huge file transfers, find a computer with gig ports or replace any intermediate switches.
Bottlenecking at 100 Mbps indicates either a slower network interface on the device in question or an older CAT5 ethernet cable, or a defective one.
As we have no idea what kind of device you have, the first thing I'd do is swap out the cable with a known good CAT 5E or better cable and see if it fixes the problem.