Well, this one is pretty strange...
Upgraded from VDSL to fibre. No changes made to my Win10 computer at all as far as it's NIC configuration.
On VDSL I was getting 29mbps upstream from my computer. After switching to fiber I've actually been downgraded to a fixed 100/20 connection, but I ONLY get 13-15mbps upstream. My downstream comes in at 105mbps.
Here's the real head scratcher.....when I test from my iphone which uses 802.11ac, I get exactly the full 20mbps. I've tested this repeatedly for the last day and it is 100% repeatable. iPhone gets 20mbps and Windows desktop and laptop gets 13-15mbps on ethernet.