Intel I210 Gigabit ethernet running incredibly slow over LAN and WAN

[frekin]

Windows 10 64 bit
Intel Ethernet I210-T1GbE NIC install in PCI-express slot
Cat 6 cable from AC1900 router reports GB connection
Updated to latest Intel NIC drivers, all driver diagnostics report good/excellent for cable and NIC

- Installed 802.11N USB WiFi adapter to check, and OOKLA reports ~165 Mbps download (rated Xfinity speed)
- With WiFi disconnected and Cat6 connected to router, router light indicates GbE connection, driver properties report 1.00 Gbps Full Duplex

All speed tests (OOKLA and 2 others) show download speed hits a hard limit of 5.8 Mbps, which corresponds to my recently observed ~700MBps download speeds.

Nothing has changed (drivers, etc) recently, except I upgraded to Windows Fall Creators Update recently.

I have tried other cables with no change, and I don't have any other LAN connected computers to try LAN xfer rates. This is a hard limit on my download bandwidth, most likely related to some setting in Windows. It's really perplexing. If I disconnect the ethernet cable and switch to my USB WiFi NIC, I get all rated xfer speeds. What am I missing?

 

[SugarPsycho]

If you think it is your windows you can download a linux system on a USB drive and run that system from the USB as a live image (I recommend Fedora or Ubuntu). If you continue running into problems Xfinity might be throttling you, Comcast is not a very reputable company everyone I have talked to has had to switch from them (mainly streamers).

Edit: I recommended the linux download to test your theory, not as a solution by the way.