This is a driver issue
I suspect it is something more complicated than that. I don't know what yet, but all I can share is my testing results which seem to prove something else is going on.
My co-worker has a Surface Pro 3 that came with Windows 8.1 on it. She's had it for a while, and just upgraded to Windows 10 last week after doing a factory reset on the OS (while still in 8.1). I asked to borrow the device to do speed tests on my home network
which is causing me problems. It is the same model (SP3, i5, 128 GB). The only main differences I can think of is the age, hers shipped with 8.1, and it has a Hynix-branded SSD instead of a Samsung model.
I brought it home, joined the 5 GHz network, and ran speed tests on both my ISP's speed test page and on speedtest.net. Sure enough, it was getting over 140 Mbps down and 11 Mbps up, which is what I would want to get on mine. Nothing else appeared to be
installed besides what came with the Surface. Like I said, it was an upgrade to Windows 10 on top of a freshly-reset 8.1 install.
She has the same driver installed with the same brand/model of wireless NIC. I wouldn't have believed it until I saw it with my own eyes.
I've tried all kinds of things to resolve this including resetting and re-configuring my router. Tonight, I'm going to try doing a factory reset on the SP3 and see what happens. I've seen others not have success, but, who knows, maybe I'll get lucky.