I have Gigabit service at my home office and noticed degradation on one of my desktop PC's. Sitting next to each other, two identical Dell machines, one gets 885 down/ 42 up, the second troubled machine gets 46 down/ 42 up. Here is a list of steps I have taken so far with no fix.
Reboot Comcast Gateway
Changed the ethernet connection on the gateway to a blank port
Swap ethernet cables between good and failing machines
Download and apply new device driver for ethernet NIC
Shutdown firewall and AV
Update FireFox and Chrome
Run Microsoft and Dell diagnostics
Changed the IP from DHCP to static
Manually changed the DNS servers to 220.127.116.11., 18.104.22.168
Only thing I have not changed is the MAC in the NIC. I even took a laptop and used the ethernet cable from the failing machine and it got full speeds.